Translated by Robert S. Root-Bernstein from Nouvelle découverte touchant la veüe (Mariotte, Oeuvres, Leide, Van der Aa, pp. 496-534)
It is necessary to see this dispute in all of its breadth to see it in all of its beauty. An extract would conceal an infinity of fine and ingenious reflections, the very delicate detail, in which all of the subtlety of the issues consists. It is only a matter of deciding which of two membranes, attached one to the other, and each as thick as a sheet of fine paper, is the organ of vision. It would seem that the difference between the two opinions would be just as small and just as inconsiderable as the distance between the two membranes, for whichever one takes in the end, one hardly errs. Readers will be all the more surprised to see sentiments so essentially opposed and a combat so soberly stubborn in a subject so slight in appearance.Histoire de L'Académie Royale des Sciences, Vol. I, 1730, pp. 102 - 103.
With regard to my Observation concerning the defect of Vision that occurs when the Picture of an Object falls exactly on the Optic Nerve, I will tell you that a long time ago the curiosity to know whether vision is stronger or weaker at the site of the Optic Nerve led me to make a curious observation which I did not expect. I took it for granted that Vision was formed by the reception of Rays which form a Picture of Objects on the fundus of the Eye, and that this Picture was in a reversed position, opposite to that of the Objects it represents. I had also often observed upon the anatomy of Men as well as of Animals that the Optic Nerve is never located exactly in the middle of the fundus of the Eye --that is to say, at the site where the Picture of Objects one looks at directly is formed -- and that in man it is a bit higher and on the side leading toward the Nose. Thus, to make the Rays from an Object fall on the Optic Nerve of my Eye, and to test what would happen, I attached a small circle of white paper to a dark background to serve as a fixed point of vision at about the level of my eyes. At the same time, I fastened another circle beside it to my right at a distance of about two feet but a little lower than the first so that it could strike the Optic Nerve of my right Eye while I held the left closed. I placed myself opposite the first paper and moved away from it little by little, always holding my right Eye fixed upon it. And when I was at a distance of ten feet, the second paper, which was about four inches wide, disappeared entirely. However, I could not attribute this to the obliquity of this object, all the more so as I noticed other objects that were even more to the side, so that I could have believed that someone had subtly removed it, if I had not found it again by moving my Eye ever so little. But as soon as I stared at the first paper, the other which was to the right disappeared instantly, and to find it again without moving the Eye, it was necessary to change position slightly. I thereafter made the same experiment at other distances, separating the papers or moving them closer together proportionately. I did it again with the left Eye, holding the right closed, after having transferred the second paper to the left of my point of vision, so that according to the position of the parts of the Eye, there is no room to doubt that the defect of Vision is made on the Optic Nerve. And it is very surprising that, when, in this manner, one loses from view a circle of black paper attached to a white background, one does not perceive any shadow or darkness at the spot where the black paper is, but the background appears entirely white.(1)
I communicated the discovery of this defect in Vision to several of my friends to whom the same thing happened, but not always at the same distance. I attributed this diversity to the different locations of their Optic Nerves. The Reverend Father de Billy was one of the first with whom I shared this experiment. You have done it yourself in the Bibliothèque du Roi, where I showed it to the Gentlemen of your Assembly. And like me, you noticed this diversity, having had there some persons who, at the distances that I said, lost from view a paper eight inches wide, and others who did not cease to see it until it was a little smaller. This can only come about from the different sizes of the Optic Nerve in different Eyes.
Thus confirmed, this Experiment has since given me cause to doubt that Vision is formed on the Retina as I had believed according to the most common opinion, and made me conjecture that it was formed rather on the other membrane that one can see at the fundus of the Eye through the Retina, which is called the Choroid. For, if it was formed on the Retina, it seems that Vision would occur wherever this Retina is found, and, since it covers all of the Nerve, as well as the rest of the fundus of the Eye, there would be no reason why Vision would not be formed at the site of the Optic Nerve where the Retina is. On the contrary, if Vision occurs in the Choroid, one will see clearly that the reason why Vision is not formed at the site of the Optic Nerve, is because this membrane begins from the circumference of this Nerve and does not cover the middle as it does the rest of the fundus of the Eye.
You know the other reasons that I have adduced in a Paper that I left
in your Assembly and that you can review, which make me conclude in favor
of the Choroid rather than the Retina. I would be pleased if you would
freely tell me your feelings, being as you are not one of those who wishes
to give conjectures in the place of demonstrations. I continue to do research
on this matter. If I come across anything worthy of your notice, I will
let you know, etc.
I received with much pleasure the Letter that you did me the honor of writing on the subject of your Observation, concerning the defect of Vision that occurs when the Picture of an Object falls exactly on the Optic Nerve. I announced it to our colleagues [nos Curieux] who were very satisfied. Everyone was surprised to see that no one before you had perceived this defect of Vision, which everyone has experienced [expérimenté] since you gave us knowledge of it. For, when we look at a Star, we often lose from view another which is to the side, and we even lose the entire Moon, which would still disappear even if it were much larger than it is. Sometimes chance makes one find that which one does not look for; I am indebted to it for many novelties. But there are few men who find novelties as you do, by looking for them: for that it is necessary to have a genius like yours and eyes as clear-sighted as you have.
I have read your opinions concerning the Choroid. I have examined the reasons that lead you to believe that this Membrane is the principal organ of Vision. I have even reread the Paper that you left in the Bibliothèque du Roi before your departure. And, I have found nothing that appeared convincing enough to me to abandon the side of the Retina. Since you wish me to tell you my thoughts freely, I ask you to receive them as a consequence of my sincerity and the desire that I have to seek the truth.
In order to take from the Retina the preference one ordinarily gives it as the principal organ of Vision, you say in your Paper, "that it is transparent and that it can receive only a very slight impression from Light, no more than diaphanous bodies such as Air and Water; and that, on the contrary, black and opaque bodies, such as is the Choroid, are easily heated by Light."
I agree that the Retina has some transparency. One can see the colors of the Choroid through this Membrane when it is contiguous to it. But there is no comparison to be made with Air, nor with Water; the transparency of the Retina is almost like that of oiled paper, and a little less than that of the ho-rn that serves for lanterns. It is white and its whiteness renders it opaque enough to stop the species [espèces] of Objects as much as is necessary for Vision. This could not be made distinctly enough on the Choroid through the Retina. For if it were necessary that the species pass through to the Choroid, the opacity of the Retina would be as harmful to Vision as if it had encountered a similar opacity in the Cornea, in the Lens, or in the other humors of the Eye which Nature has made diaphanous to allow the species to pass through to the organ of Vision.
It is easy to observe the opacity of the Retina. It is necessary to have a very fresh eye, to cut away gently the Sclera and the Choroid, to lift them adroitly away, and to leave the Retina extended over the Vitreous Humor; and now, one cannot see very well through this membrane.
The opacity of the Retina is also apparent when one plunges it into water, for there it appears completely white and almost without transparency.
The blackness of the Choroid, which you judge to be necessary for Vision, is not found equally in all kinds of eyes. One finds it, to be sure, in the eyes of men, but the degree of blackness is different among diverse individuals. Where this blackness is found in the eyes of birds and in some other animals, the diversity is the same. But in the eyes of Lions, Camels, Bears, Cattle, Deer, Dogs, Cats and many other animals, we often see colors as lively as those in Mother of Pearl and in the Iris. This [the Iris] forms a sort of curtain around the base of the Choroid at the place most exposed to visual Rays. And when we gently scrape away these colors with a scalpel, we discover a white substance with which this part is coated. The Iris is coated with the white substance in such a manner that it does not permit the species of objects to pass through to the blackness of the Choroid to make the impression that you demand in order to produce Vision there. And it seems that the black color has no other purpose than that of preventing light from entering the Eye from any other place than by the hole in the anterior uvea, that is to say, by the Pupil. For if this blackness were not in the Choroid, as a curtain behind the entire Sclera, the Light would enter through the Sclera, as through a parchment, and, going to the base of the eye, would efface the species of Objects, preventing the formation of Vision by this means.
Fish also have a very bright color at the base of the Choroid, but of another sort. It appears as if it is made of polished silver or the luster of Oriental Pearls.
This variety of colors is never found in the Retina. It retains its whiteness and opacity in all kinds of eyes, and it is this which persuades me that it is more appropriate for Vision than is the Choroid. For the uniformity and indifference it has for all colors gives it the facility for receiving the impression of their differences. This cannot be done by the multitude of colors that one finds at the base of the Choroid which, mixing with those coming from the Objects, could only transmit the sense of Vision with great confusion.(2)
When you say that black bodies receive a much greater impression from Light than white ones, this ought to be understood to mean when the Light is received immediately on the black body without any medium weakening its rays. Black paper exposed at the focus of a burning mirror burns almost instantly. White paper burns only with difficulty if the Light does not encounter some blackness or discoloration in it. And when black paper is placed behind the-white, it receives only the slightest impression of heat which does not burn it. But neither the impression of heat nor any other impression that Light can produce is relevant here. Light acts only as that agent which forms the distinct representation of objects. This representation does not require heat, but only a moderate Light for illuminating. This cannot be done as well on the Choroid as it can on the Retina, which forms an obstacle for the Choroid, and which itself has no obstacle before it when the Cornea and the humors of the Eye are not altered.
You assert in your Paper, "That the Retina does not penetrate into the Brain as does the Choroid which envelops the Optic Nerve behind the Eye and accompanies it into the midst of the Brain."
This statement surprised me a little, for the Retina, as you know, originates at the very extremity of the Optic Nerve, which ends at the base of the Eye in the same way a flower comes from the very extremity of its stem. It is composed of strongly delineated Filaments which could only come from those of the Nerve. These Filaments easily appear when one plunges this Membrane into water, for they are more opaque than the water.(3) They are also more opaque than the mucous Tunic which envelops them and which disappears in the water. These are the Filaments to which the name Retina has been given, if one is to believe the Anatomists. It has Veins and Arteries which slide among the Filaments and which are covered only by the mucous Tunic which holds them bound together. And insofar as this mucous Tunic is transparent, it does not prevent these vessels from being seen when they are full of blood, as if they were covered only by their own membrane.
From this composition and this origin of the Retina it is easy to judge that it must have continuity with the Brain. For, by means of the Optic Nerve, of which one could say that it is an extension, the Retina takes its primary origin from the principal part of the Brain. This is the bulge which forms the top of the medulla of the Spine, from which derive the principal nerves which serve our senses.
The Choroid does not have that advantage. It is composed, in truth, of the Pia Mater. The Pia Mater could very well give it a sensation of pain (which is common to all Membranes), but not that of Vision, which requires a different impression than that which causes pain. The Membrane that the Pia Mater gives rise to in the Choroid must be diaphanous, because the Pia Mater is itself diaphanous outside the Eye and in the Brain. Consequently, it must allow visual Rays to pass through the [blood] vessels it envelops -- which [vessels] form the blackness that one sees in the Choroid as a result of the blood they contain.(4) But these vessels, which come from the heart, have no capacity for vision, which cannot be made without communication with the Brain. These vessels take their origin from the Carotid Arteries and the internal Jugular and, passing through the Sclera (which they pierce in diverse places), they hold it as if it were sewn to the Choroid, which they render opaque and cause to resemble the Chorium, or the Placenta of a Fetus. From this the Anatomists have given it the name Choroid. Among these Veins and Arteries there are some Nerve Filaments which come from the Motor [Nerves] of the Eye, but these do not serve for Vision. Thus, I again see nothing in the Choroid that gives it communication with the Brain, whereas the Retina originates in the Optic Nerve.
You can judge well from this discourse that I have no trouble believing that the Choroid is rendered opaque by the vessels that surround it. For these vessels are within the Sclera like a very black curtain that stops Light and prevents it from passing through this delicate membrane that forms that which you call the Choroid. No more do I deny that this blackness could receive the impression of Light if the best part of the species were not stopped by the opacity of the Retina. [But] the Retina is sufficient to retain the image of Objects, as we see that it does when we look into a fresh eye and observe what passes within through a hole that has been cut in the top. For we see through the Humors that the image of Objects is painted distinctly on the anterior surface of the Retina. We see it even better when this opening is made at the base of the Eye opposite to the Pupil, so that one leaves only the Retina extended over the Humors. For in this Eye, once applied to the hole of a camera obscura, you see the images of Objects stopped on the Retina just as we see them on oiled paper. Up to now, it is this -that has prevented me from abandoning the cause of the Retina for that of the Choroid until I should be convinced by better reasons. But let us pass on to other arguments that you have used in your Paper.
You say, "that it is necessary, in order to form distinct Vision, that the Rays which come to the Eye from each point on the Object unite in a point on the Organ. Since the Retina is a demi-ligne thick,(5) if the rays unite on its surface contiguous to the vitreous humor, they will intersect and fall on diverse points on its other surface contiguous to the Choroid. And if they unite on this other surface, they will intersect and fall on diverse points on the former. If they unite between the two surfaces, within the thickness of the Retina, they will fall in diverse points on both surfaces. In all these ways the Retina will form confused Vision, whereas the Choroid, being very delicate and opaque, can receive in one point the Rays from one luminous point."
I agree with you "that it is necessary in order to form distinct Vision that the Rays,which come to the Eye from each point on the Object, unite in a point on the Organ." But you must also agree with me that this is not a Mathematical point, but a Physical point,which has size and even considerable size, since in the smallest part of an Object that our eyes can see, we discover many other parts with the Microscope. We can say, then, that we see the Object much more distinctly than before, although it did not appear to us in any way confused. From this it is evident that, if a point falls on the Retina, it will cover as much space on the surface of this Membrane as it is in size; and that if it falls on the thickness of the Retina, it will occupy at least all of this thickness, especially in the eye of a man where the Retina is hardly thicker than a sheet of common paper, of which it takes almost twenty thicknesses to make a ligne. For, when you say that the Retina "is the thickness of a demi-ligne," I do not think that you intended to speak of the Eyes of Man; and I do not know in what animal it has such a thickness, since Cattle, Horses, Deer, Lions, Bears, Wild Boars, Pigs, Sheep, Dogs, and other large Animals, which have come to my knowledge, do not have a Retina thicker than three or four sheets of paper, which does not make a quarter of a ligne. Thus I do not see that any of your Discourse to this point can strike a blow at the opinion of those who hold that the Retina is the principal Organ of Vision.
But if I were to agree with you that the Retina has as much thickness as you give it, this thickness would serve only to render it more white and opaque and to allow fewer rays to pass to the Choroid, which becomes in this way less appropriate to be the Organ of Vision.
You go on to say, "That the Choroid, being very delicate and opaque, could receive in one point these Rays from one luminous point." I would not doubt this, even if the Choroid were very thick, if the Retina, which has great opacity, did not form an obstacle for it: but I am convinced that this opacity of the Retina can stop the image of objects and prevent it from passing (if not, perhaps very weakly) to the Choroid. And I cannot depart from this belief until you have demonstrated the contrary. But let us come more forcefully to your Argument founded in the lack of vision which occurs in the experiment that you have demonstrated to us.
"It follows," you say, "from this experiment that since Vision occurs everywhere that the Choroid is and that there is no Vision where the Choroid is not -- despite the Retina being there -- the Choroid must be the organ of vision and not the Retina."
I know that the Choroid is not extended over the extremity of the Optic Nerve. It is pierced at the base of the Eye in order to allow the entrance of this Nerve, and thereby give rise to the Retina, which is only an effusion of Filaments from the Nerve. These Filaments are enveloped by a mucous membrane that is moistened by the vessels that also come from the same Nerve or from its circumference.
I conceive the effusion of these Filaments at the end of the Nerve to be like those Fibers that go out from the stem of a plant and spread themselves out to all parts to form a flower at the end of the stem. And I conceive in the midst of this effusion of Filaments, a point that must be its center; this is the same as that in the middle of a powder hoop(6) which has been knocked over and whose fibers are scattered in all directions: there is a point which is the center of all the fibers.
This effusion of Filaments which compose the Retina can be conceived of in two ways. The first is to imagine that all of the Filaments that are closest to the center of the Optic Nerve radiate out to end precisely at its circumference, after having equally covered it by effusing themselves in all directions. None of these Filaments terminate in the expanse of the Nerve, but end at the Nerve's circumference. The other Filaments of the Optic Nerve radiate further out into the expanse of the Retina, in proportion to their distance from the center of the Nerve at the point where they leave it. Thus, all of the internal surface of the Retina is composed of the endings of these Filaments,with the exception of the area at the end of the Nerve, where no filaments end. If one thinks, as some of our modern Philosophers do, that the Vision is made only when the visual Rays fall on the extremity of one of these Filaments, then one could correctly explain your experiment [expérience] by this conception, for the area at the end of the Nerve has no Filament endings from its center to its circumference. Thus, this area receives no impressions from the visual Rays necessary for Vision, except at its circumference, and this will be the cause of one not being able to see an Object whose species fall inside this circumference. But since this configuration of the Filaments of the Retina is perhaps an effect of imagination as much as of Nature -- there being few reasons not to end a Filament between the center of the Nerve and its circumference, and even in the midst of the center -- I do not see enough certitude in this position to be obliged to follow it.
The other manner in which to conceive of the effusion of the Filaments of the Retina is to imagine them radiating to all ends of this tunic's expanse, like the fibers of a Plant to the extremity of its Flower, or like the fibers of an overturned powder hoop stretching out from the hoop to its extremities. In either case it is necessary to imagine a point in the middle of these Filaments from which they begin to scatter, and to conceive some depth there similar to that which one sees in the middle of the hoop. And if one considers in what fashion the visual Rays fall on this deep point and its environs when one does your experiment [expérience], one will find that they fall [there] in another manner than on those places on these same Filaments where Vision occurs. For the rays strike the latter filaments directly,whereas none at all will strike those Filaments that are in the environs of the deepest point, or they will do so obliquely. This could cause the defect in Vision, principally when the object is not very luminous, because those objects that are not very luminous (like a candle which one sees at a distance of four or five paces) are lost so absolutely that one cannot perceive their light.
But there is something else at the place of the Optic Nerve that appears to cause the loss of Objects. This is the blood vessels of the Retina whose trunks are large enough to create an obstacle for Vision.
These vessels, which are only the branches of the Veins and Arteries, draw their origin from the Heart, and,having no communication with the Brain, cannot carry there the species of Objects.
Thus, if the visual Rays originating from an Object fall on the trunks of these vessels, it is certain that the impression that they will make there will produce no Vision and that the picture of the Object will be formed defectively. This is similar to what happens on white paper in a camera obscura when there are several black spots or some holes of considerable size in the paper: for the more sensible this blackness or these holes are, the more they rob the image of the Object from our Eyes.
It is not the same with regard to the small branches that depart from these trunks,since the branches spread themselves out in the Retina. When they join (as happens frequently) at the fundus of the Eye where the most distinct Vision is formed, they make no defect in the image of the Object because they are so small. This is the same as our mirrors when they are missing some lead or tin in a place large enough to be perceived; then the image that we see appears perforated. This does not happen when there are only small holes such as those made by the point of a needle.
I know well that the impression that an image makes in the Eye on the Retina, or on a white paper in a camera obscura, is quite different from the one that we see in our mirrors. For the image paints itself on the surface of the Retina and the paper as if it were a true painting that one always sees in the same place from whatever angle one looks at it. But the image does not paint itself at all on the surface of our Mirrors, it paints itself only in our eyes and appears as far behind the glass as the Object which sends its image to this glass is distant [from the Mirror]. From this it is easy to determine that Mirrors receive no impression from the visual Rays,since the Rays do not stop there,but are only reflected. Thus the object is seen only by these reflected Rays, which carry the image to the Eyes.
Thus each time the species of an Object fall on the trunks of the vessels of the Retina, they will be lost without a doubt, in proportion to the size of the trunks. And this loss will create, in the total image, a defect that will appear at a greater or smaller distance from the paper, which you establish for the fixed point of your experiment, in proportion to the distance of the trunks of the vessels from the axis of the Rays that fall at the base of the Eye at the spot where the best Vision is formed. [However,] it is certain that the vessels are not always equally distant from this Axis in different Eyes, for often these vessels enter the Retina at the center of the Nerve, sometimes at the circumference, and sometimes also at the area that is between the center and the circumference. And this could well be the reason for that which we have noticed; that it is necessary to keep the paper that one loses from view at a greater or lesser distance according to the diversity of people doing this Experiment. For some lose the paper [when the papers are] two feet [apart], others at less than two feet [apart], and others at a greater distance. Some lose it a little higher and others a little lower according to where the trunks of the vessels are situated with regard to the Optic Nerve. And some lose it more than others according to the greater or lesser size of the vessels, for their size is as different as the temperaments of the individuals. And, because it is difficult to determine precisely the place where the Objects are lost in all sorts of Eyes, we have reason to believe that this loss is not always made on the area of the Nerve where the Retina is, but sometimes made outside of this area where the Choroid is found. For the trunks of the Retina's vessels are so large and so long as to extend themselves on all sides of the Nerve and by this means to cover part of the Choroid in proportion to their size. And in that case it will be true to say that vision is not made in all the places that the Choroid is found, although it is exposed to Light: this strikes a blow at your opinion because you cannot doubt that these trunks prevent the species of Objects which fall on them from reaching the Choroid, and that the image would be defective in this place since the species would not be able to make an impression on the organ of Vision through these vessels.
There, Monsieur, are the principle reasons for my doubts concerning the organ of Vision. I acknowledge to you that your experiment would already have settled me in favor of the Choroid if these vessels in the Retina and the opacity of this Membrane did not yet hold me in suspense. But I cannot drop the common opinion to embrace another which has not been demonstrated and which exists problematically. I hope that you will give me some new light that will convince me, [since I] have all the inclination possible to follow your opinions, to which I will always defer with respect.
A beautiful discovery like yours could not fail to be quickly confirmed because the secret of your Experiment is to make the picture of the Object fall just on the Optic Nerve or close to that Nerve. MM. Picard has noticed a way in which one can lose an Object while holding both eyes open at once. The reason for this is that one makes the image or the picture of the Object fall on both Optic Nerves at the same time. And this is how:
It is necessary to attach a round piece of paper, A, an inch or two in size, to a wall and to make two marks, B and C, on the wall beside the paper, one to the right and one to the left, each at a distance of about two feet. Then place yourself directly in front of the paper at a distance of nine or so feet and put the end of your finger in front of your two Eyes, as D, so that it hides the mark made to the left of the paper from the right Eye and the mark to the right from the left Eye. If one remains motionless in this position and stares at the end of one's finger with both eyes, the paper, which is in no way covered, disappears entirely. This is all the more surprising since without this particular meeting of the Optic Nerves, EF, where no Vision is made, and on which the Rays AE and AF fall when one loses the view of paper A, the paper would appear double, as one proves each time the finger is not placed as it must be, or each time that vision occurs so as to be a little to the side. The reason for this you know well enough without the necessity of explaining it here.
The application of this procedure to yours is easy. For when one looks
fixedly with both Eyes at the end of one's finger which one places between
the marks, it is the same as if one had pointed each Eye at the particular
spot at which it is necessary to look to lose the paper: thus one can do
with two Eyes the same thing as that which you do with one while holding
the other shut, etc.(7)
Monsieur, I have seen in your Response the reasons which prevent you from believing that the Choroid is the principal organ of Vision. Although they have much subtlety and plausibility, I have not found them compelling enough to oblige me to render the advantage to the Retina.
You say in your first objection that, "if one lifts the Sclera and the Choroid away from a very fresh eye, leaving the Retina extended over the vitreous humor, then one cannot see very well through this Membrane." From this you concluded that it does not have enough transparency to allow sufficient light for Vision to pass to the Choroid. I do not agree with this consequence since there may be a great deal of difference between the Retina of a dead animal exposed to the air and that of a living animal properly enclosed between the vitreous humor and the Choroid. Different dispositions ordinarily change the qualities of things: fat, which is transparent when melted, becomes opaque when cooled; the cornea of an eye which is kept for several hours in warm air becomes disturbed and, little by little, entirely opaque. But so that you may be persuaded that the Choroid receives enough light [for Vision] in the living animal, it is necessary to take an eye still warm from a freshly killed Cow and to cut it in two a little below the Lens so that a good part of the Vitreous Humor stays extended over the Retina. Then you will see distinctly the diverse colors of the Choroid, the base of the Optic Nerve, the trunks of the little vessels which originate there, and their branchings in the thickness of the Retina. [These will appear] with so much clarity that you can even discern if the Retina extends on all sides of the Vitreous Humor. From this you may determine that the light which objects send to the Choroid is more than sufficient to produce Vision, since coming to your eyes by reflection and by a second passage through the Retina and the Vitreous Humor of the cut Eye, it is still strong enough for you to see clearly and distinctly the Choroid itself.
This is not to say that I deny that the Retina has some whiteness in the living animal and that it may be a little less transparent than other humors, principally in the part contiguous with the Choroid. Nature could have made it thus to subdue the brightness of great lights and to exclude their dazzle, just as she has extended an insensible Epidermis under our skin to prevent it from being too easily wounded by Bodies which we touch and by excesses of heat and cold. But, if I were to deny absolutely that the Retina has any opacity in a living animal, then your experiment would not convince me anyway, since it is made only on a Retina whose most subtile and transparent parts have evaporated. I would give as an example a white [sheet of] paper when it is wet, through which one can see Objects which are contiguous to it very distinctly: it regains its original opacity after it has been exposed for a short time to the air. And, if this example is not sufficient, I adduce the small Lens which is found in the midst of the large Lens of many animals, and which is like a kernel [within the larger]. This small Lens, being as transparent as the other humors of the Eye in the living animal, becomes white and opaque two or three days after death, although it remains enclosed in the Eye. The exterior Lens remains transparent.
Your second experiment for proving the opacity of the Retina, which is to submerse it in water, is again extremely deceptive. For you do not doubt that the Hyalin Membrane which envelops the Vitreous Humor is perfectly transparent. However, if you put a portion of the Vitreous Humor and part of the Hyalin Membrane to which it is attached in some water, although the Vitreous Humor preserves its transparency there will appear a whiteness and disturbances like the web of a spider. Plunging it into water is not a good proof for determining whether the Retina is opaque in a living animal. And as for any proof that you would put forth after having exposed the Retina to the air, [I say that] you cannot draw any consequence from this to prove that it is opaque in its natural state because the Lens itself becomes a little clouded in water,and if one leaves it there for some time or exposes it to the frost, it becomes as white and as opaque as the snow.
Thus, to resolve our differences and to know with certainty whether light from Objects passes almost completely through to the Choroid, or whether it is almost completely stopped by the Retina, it is necessary to adduce observations made on the Retina and the Choroid when they are in their natural state, as in the following observation.
At night, put a lighted candle very close to your eyes and make a dog look at you from a distance of eight or ten feet. Then you will see in its eyes a very bright light that I maintain proceeds from the reflection of the light of the candle, whose image is painted on the Choroid of the dog. This Choroid, having much whiteness, makes the reflection very strong. For, if it proceeded from the Lens or the Retina, one would see the same effect in the eyes of men and those birds and other animals which have a black Choroid.
It is therefore manifest by this experiment that the light rays pass to the Choroid with a great deal of force and that the Retina receives very little impression. And here is how the image [in the dog's eye] is created: the small picture of the candle that is on the Choroid (this being the focus of the Lens and the other humors together) sends forth rays across these humors, which reunite reciprocally with the candle. In consequence, eyes that are close to the candle must see the Lens of the dog greatly illuminated. Opticians know this demonstration and those who do not know Optics will be able to see an extremely similar effect by a very simple experiment.
It is necessary to place a spherical bottle full of very clear water eight or ten feet from a candle and to put a white paper behind the bottle at a distance of about half its diameter so that the light of the candle which passes through the bottle is reunited in a small area on the paper. Then those who have their eyes close to the candle will see the bottle full of light. This luminescence will disappear if one moves the paper closer or further from the bottle. If one puts a [second] lighted candle in place of the paper, and one holds one's Eye in the place of the [first] candle after having removed it, one will see the bottle even more illuminated than before. And one will easily be able to determine that the light which appears in the Eye of the dog results from a similar cause. You may confirm this idea if you place a candle to one side so that your face is strongly lit and so that the candle does not shine at the dog. You will then see a great deal of light in the dog's eyes. But, if you put some opaque body in front of your face to cover the candle, this light disappears. Thus you will know manifestly that the light which appeared in the dog's eyes proceeded from the image of your face and that it is painted on the Choroid for the reason I have given above. One could do the same experiment in the eyes of many other animals and particularly in the eyes of cats where this luminescence appears to be bluish. This demonstrates that the light comes from the Choroid which has a great deal of this color. But neither this color nor any other which is in the Choroid causes any confusion in the sense of vision since the senses receive no impression except from their proper organs.
The rest of this first objection has almost no other basis than an interpretation, contrary to my thinking, that you have given to some of the words of my Paper. For though I have said that only black or opaque bodies receive much impression from light, I did not mean both opaque and black together. It would suffice to say black bodies since all black bodies are opaque. But my thinking on this subject had been, and still is, that transparent bodies like air, water, and the Retina in living animals receive little impression from light and that opaque ones receive a great deal. But further, black bodies receive more than other opaque ones, and air and water a little less than the Retina. Also, I believe neither that blackness is absolutely necessary for Vision in the Choroid (only for a stronger Vision) nor that the painting of Objects must be expressed there. It suffices that the rays from each point of an object reunite in a distinct and separate point according to their proper correlation. And you will remain in agreement that since a strongly convex glass lens makes the image of the Sun appear in a united form on a white paper with a great deal of clarity and light while on a black paper very obscurely, it is on the black paper that the fire takes first since it receives much more impression than on the white. Thus, the rays from illuminated objects reunited by the medium of the Lens [of the Eye] on a whitish Choroid form a visible painting there, and a very obscure painting on a black Choroid. But again, the impression is much stronger on the black than on the white. And this is the reason why men and birds see better and more distinctly than most other animals: their Choroid is black and, in consequence, very sensitive to light. They can greatly contract their pupil, thus allowing that tile rays that pass through the pupil from each point of the objects all be reunited more exactly in a point than in most animals that have a whitish Choroid. As a consequence of having a whitish Choroid around the axis of vision, such animals are less sensitive to light and must compensate by dilating the pupils of their eyes since they need a greater illumination. This prevents their vision from being distinct, because the rays which fall on the extremity of the Lens are bent more in their refraction. It is true that, in order to rectify this defect in some fashion, they have a small Lens in the midst of the large and that this small Lens is of a thicker consistency than that of the large Lens. Thus, the small Lens has a stronger refraction so that the rays which come from a point outside the Eye, falling on the Lens close to the axis of vision, are bent more in passing through this small Lens. By this means the rays are better reunited at the fundus of the eye with the rays that fall on the extremity of the large Lens. This renders the vision [of such animals] less confused, although never as distinct as in men and birds who have only one Lens. Fish also have a double Lens because otherwise their vision would be even more confused than that of animals that live in the air, for the rays cut the axis so unequally in their spherical Lens that it would act as if lenticular. And,if the Lens were not spherical, their focus would be very long since the refraction of the rays that pass from water to the Lens is very small.
The difficulty with your second objection comes again from an equivocation, and consists in not knowing what one must cite in order to maintain a greater continuity and communication with the brain. My hypothesis is that the nerves are all covered by the Pia Mater, which envelops the spinal chord and has with it a continuity of Fibers, such that, however little the nerves are moved, the impression is carried from there to the brain by this continuity of Fibers. And whether the tissue is different in the nerves of the different senses or whether the nerves contain some spiritual liquors that determine their sensations by their differences, it is certain that the nerves of vision, in whatever manner they are moved, represent colors and light; those of hearing, sounds; and those of touch, pain; etc. Now, the Choroid is an effusion and dilation of the Pia Mater,which envelops the Optic Nerve and which comes, by a continuity of Fibers, from the tuberosity of the spinal chord that is in the brain. From this it follows that no matter how lightly the Choroid is touched, the impression can easily be communicated to the brain. And in order for one to be able to say the same thing of the Retina, it is necessary that there be a small canal in the Optic Nerve by which the Retina extends its proper substance through that tuberosity by a continuity of fibers: one does not see this and you were constrained to say "that there are small filaments of this Nerve which have this continuity." But if there were these filaments, they would spread throughout the Retina from the center to the circumference. Yet, if one looks at the Retina with a Microscope, one never sees any filaments at all, but [rather] it appears to be of a uniform consistency like the vitreous humor and must be considered as a fourth humor flowing and passing through the pores of the Optic Nerve and containing no filaments. It is true that if one passes a pin through the thickness of the Retina, one often encounters some filaments. But, if one looks at them through a very convex lens of glass, one discovers that they end on the small vessels of the veins and arteries which are in the Retina. And infallibly, if there were small nerves, one would encounter them and they would stop the pin since they would be as strong as small arteries. When you say that you distinguish these filaments in water because the rest of the Retina disappears, this repudiates experience, for you have said beforehand that one sees the Retina all white and without transparency in water. It is up to you and those of your opinion to find good enough Microscopes to make these filaments visible: otherwise, one must believe them to be something invented for [your] pleasure.
You next adduce two experiments. The first is that if one makes an opening in the top of an eye, one can expose the painting of objects on the anterior of the Retina. But, if by the top of the eye you mean the cornea or the white of the eye to which it is contiguous, the aqueous humor will flow out by the opening you make there and the cornea will be wrinkled, which will preclude the picture from being distinct. Further, he who looks in by this opening will preclude rays of objects from passing to the eye so that he will be able to see only his own image there. If you mean that one removes the cornea entirely, the same inconvenience will occur, and there will no longer be enough distance between the Lens and the Retina to make the picture distinct. Finally, I do not believe that one can come to the conclusion of this experiment, still less be able to discern if it is on the anterior surface of the Retina or on the posterior surface that the picture is formed, since it is less than half a ligne in thickness. There is reason to believe that you have trusted in the reports of others on this experiment or that you have supposed that the images that appear in the eyes are painted on the Retina rather than that they proceed from the reflection that is made on the exterior of the cornea.
The second of your experiments is acceptable and easy to perform, but according to you, it should be impossible. For, since you hold that it is the anterior of the Retina on which one sees the picture and elsewhere have said that it is not possible to see through this membrane, it follows that one would not be able to see the picture across the thickness of the Retina. But because I believe that the part of the Retina that is not exposed to the air still retains sufficient transparency, I do not doubt that the picture could be seen on the posterior part. For when the Retina itself would have been removed and only the vitreous humor would remain, one would be able to see an inverted picture of windows toward the circumference of the vitreous humor while holding this eye at the back of a room. In the same manner one sees this picture in the front of a spherical glass bottle full of water, although one seems to see it on the exterior of the glass: this entirely destroys the consequence you wish to draw therefrom.
In the third of your objections you quoted what I said a little differently than the way I said it. For I had put in my Paper that the Retina has a thickness of about half a ligne and not a half a ligne exactly. But if it has a thickness of only a quarter of a ligne, or still less, it still has sufficient thickness for the effect I attribute to it, and for another of which I have spoken in my Paper. This is that the rays from the same luminous point which are not reunited precisely in the same point on the axis are rectified by the concavity of the Retina; those further away from the axis more than those which are closer. This makes it so that they reunite better in one point on the Choroid. This point I hold with you to be a Physical point since the objective point is one as well. But I hold that it is a much smaller point than any that could be perceptible to the Vision because we distinguish the diverse parts of very small objects (like the extremities of the larger of the small arteries in the Retina) that do not have the eighth part of its thickness. And that which represents this small size in the organ of Vision is not equal to it, as you pretend, but it must be twenty-five or thirty times smaller. This may be determined from the ratio of the distance of the object to the center of Vision and the distance from this center to the organ of Vision. In consequence, the thickness of the Retina is not proper for the small [size of the points of Vision].
Thus you see, Monsieur, that to this point your objections have struck no blows at my opinion and that the transparency of the Retina has been well established. We come now to the proof that I take from the defect in Vision on the base of the Optic Nerve.
It is first necessary to agree that in this experiment almost all men completely lose from view the circle of white paper whose diameter is the ninth or tenth part of its distance to the eye. Now, the visual Triangle, of which the diameter of this paper is the base and whose apex is the center of Vision, is proportional to the Triangle whose base is the diameter of the picture of this paper on the back of the eye and whose apex is the same center of Vision. This center is removed six or seven lignes from the base of the Optic Nerve, which has a size of about three-quarters of a ligne. This base is about a ninth or tenth part of the distance to the center of Vision. And by the principles of Optics, the image of the circle of paper on the base of the Nerve will cover it exactly, [so that], if the paper disappears entirely, it follows that the entire base of the Optic Nerve is insensible to light. From this I conclude that the Choroid is the principal organ of Vision, since its absence causes the defect in Vision, and that the Retina is not, since it is found in this place, and seems to be disposed there the same as in the rest of the fundus of the eye.
In order to elude the force of this argument you adduce other causes for this defect of vision: the first two are almost the same, but it seems to me that you suppose them without basis. For, as was said before, one finds no nerve filaments leaving from the base of the Optic Nerve and these would not be appropriate for vision, since they would leave very large gaps in some parts of the Retina. It is necessary that each point from an Object meet one sensible point on the organ of Vision so that the rays may reunite there. This [structure] is found in the Choroid, which is the effusion of the sensible part of the nerve in a continuous membrane. Besides, the causes of the defect in vision cannot be found in these hypotheses [of yours] because, in the first instance, what reason could one give for there not being any extremities of these filaments opposite the Optic Nerve, since this would require only the simple, direct continuation of some of these fibers to the anterior part of the Retina?
And as for the second hypothesis, which is your special belief, I agree that the void of your inverted hoop could cause the defect in vision at the center of the base of the nerve, but I do not see why these filaments, which cover the rest of the fundus according to you, would be insensible to light in this place. It is not necessary for vision that the rays fall directly on the organ of Vision, and it suffices that those of the same luminous point reunite in a single point. Moreover, it is easy to determine that there is only a single ray of those which converge to a point (be it on the Retina or on the Choroid) that can fall directly [i.e., at right angles] there. But I will not expand more on this subject, since I believe that this inverted hoop and the filaments which compose it are something without basis and that only you know how to make them [the filaments] visible.
The other cause that you adduce is the trunks of the vessels that leave from the base of the nerve. But you cannot deny that they are very small and that one has some trouble in discerning where the small bundles pass through [the nerve] when one cuts the nerve very close to its insertion into the eye. And the vessels often leave the base by two different small bundles whose diameters occupy no more than the eighth part of that of the base [of the nerve]. It follows that if the rest of the base of the nerve were sensitive to light, then one would only lose a paper of at most two inches in diameter at a distance of ten feet. And sometimes in fixing the Eye on a small paper, it would disappear in two other very small, separate [areas] as well, without losing the view of what was in between. This repudiates experience. Thus, the causes that you allege for this defect of vision are insufficient, whether or not they have any foundation. It follows that the one that I offer always remains for your consideration. And to confirm my theory further, I will adjoin here some observations and some arguments that are neither in my Letter nor in my Paper.
The first observation, which is very common, is that the pupil dilates in the dark and contracts in great light. And it is difficult to find the cause of this involuntary movement unless it is supposed that the Choroid is sensitive to light, for then it is easy to determine that on being struck by a strong Vision, it could dilate or contract its Fibers which are continuous with the anterior of the Iris so that it may contract this opening; and, if not struck, it relaxes the opening. If instead, one supposes that the Retina is the organ of Vision, it is difficult to explain how this contraction occurs.
The second [observation] is that,if one holds a hand between a spherical glass bottle full of water and a candle placed in front of the bottle, one feels more heat than if one holds it on the reciprocal side, that is to say, at the place where the rays, which have passed through the bottle, make the large inverted image of the candle flame appear on a white surface opposite. For I draw this consequence: that the image of the candle which is painted on the Choroid of a dog, as I have proven, makes much more impression on the Retina of the Dog than on that of [he] who looks at the Dog and sees it very clearly. From this I conclude that, if the Retina were the organ of Vision, the Dog would not see poorly illuminated objects which were around the candle (even if they were three or four feet away) since they would receive much more of an impression from the reflection than from the objects: the greater sensation effaces the lesser. This repudiates experience and it is not true that there are such defects in the vision of animals.
The third [observation] is that the eyes of birds are disposed so that the Optic Nerve curves down into the concavity of the Sclera after its insertion into the eye and the length of this curve gives rise to the Choroid that covers it leaving only one white ray in the middle. The Retina arises from this, extending itself on the Choroid at the fundus of the eye. But it is covered next to this white ray by a small black membrane contiguous to or stuck to the Hyalin Membrane, having a length of about six lignes and a breadth of five in the eyes of large birds. This membrane also proceeds from the Pia Mater which envelops the inside of the Optic Nerve and is like an appendix to the Choroid. There are some birds, like the Ostrich, in which the Optic Nerve becomes enlarged at the base of the Eye in a thick membrane of oval shape, the extremities of which give rise to the Choroid and the Retina. But the black membrane which also arises from the same extremities entirely covers this oval and extends a little over the Retina.(8) If one considers the place where this black membrane is in all types of birds, one finds that it is a little to one side of the axis [of vision] and that the rays from objects which the birds look at with both eyes fall precisely on it. One can easily determine this if one notices that birds do not have parallel axes in their eyes. When their eyes turn toward the same object, the axes are a little skewed to one side. This makes the rays from this object fall obliquely on their Corneas, and thus -- according to the rules of refraction -- they bend close to the same axes in the back of their eyes. Now, since the Retina is not at this place, or it is covered by this black membrane which stops light (yet no one can doubt that birds are more clear-sighted than other animals), you must confess that the Retina is not the principal organ of Vision and that the advantage must be given to the Choroid.
As for the experiment of Monsieur Picard, I have found it to be very well contrived, but it is difficult [to do] because of the great effort necessary to fix both eyes on one point which is no more than four inches away. Here is another [experiment], which can be made with much less pain and which is no less surprising. Attach two small circles of white paper to a dark background at the same height, three feet apart. Place yourself in front of them at a distance of twelve or thirteen feet and hold your finger elevated in front of your eyes at a distance of about eight inches so that it hides the paper which is on your left from your right Eye and the paper which is on your right from your left Eye. Then, if you stare at your finger with both eyes, you lose both papers from view. This results from the eyes being so disposed that each of them receives the image of one of the papers on the Optic Nerve, the finger covering the other image.
One could do the same experiment with two lighted candles observed at
the same distances. If they are more distant from one another, it is necessary
to move oneself proportionately further away. That is to say, that,if their
distance apart is six feet, it is necessary to be twenty-five feet away,
and proportionately with other distances. But it is necessary that the
finger always stay in about the same position because, if one holds it
a foot or more away from the eyes, one would see four candles instead of
Monsieur, I am surprised at the novelty of your marvelous observations touching the [visual] loss of an object that one perceives when it is at a certain distance away and in a situation convenient for it to be considered by the eye. But, I still cannot share the sentiments that you have as to the cause of this accident, nor can I approve of the consequences that you draw from it in order to persuade [us] that the Choroid must be the principal organ of Vision and not the Retina as is commonly believed. Monsieur Pecquet having communicated to me the reasons that he wanted to oppose to your own in a paper he addresses to you on this subject, I reminded him of a comment that we had often made together on the Eyes of most Animals.(10) The Retina, in several places, and apparently there where the vision of objects that are seen directly is formed, is traversed by blood-filled vessels. Opaque bodies of considerable size, and interposed between the objects and the Choroid, [these vessels] must inhibit Sight if the Choroid is the true organ of vision. I do not know whether the love each has for his own ideas makes me err on this occasion, but neither do I believe that one can make a stronger objection to the use that you attribute to the Choroid, or that a more convincing argument can be made for attributing this use to the Retina [than the argument I have presented above]. The desire that I have for finding a solution urges me to write to you on this particular subject, since Monsieur Pecquet (who agrees with this fact, as it is known to him -- as well as to all the rest of our company -- by many experiments [expériences]) did not draw out the conclusions from this fact, which furnish a very reasonable basis against your opinion. And I believe that it is necessary to explain to you more distinctly my sentiments, which have not been made [clear].
My thinking is that, for vision, the species are received on the anterior surface of the Retina, which is contiguous to the surface of the vitreous humor; that this surface alone serves vision by being indivisible; that the rest of the body of this membrane, which is of considerable thickness, is necessary only for making this surface more even, just as experience teaches of plaster on walls, which could not have a smooth surface if they were not thick, according to the remark of Vitruvius who compared them to mirrors of metal which cannot be polished when they are thin; and finally that the Choroid, being covered as it is by an uneven substance resembling badly mixed, blackish mud, cannot have a polished surface and is not capable of receiving (as is necessary) the impression of rays which leave from objects.
For it must be agreed that a polish and an exact evenness of surface is a condition without which one cannot conceive of Vision being formed on the membrane which should be recognized the organ of Vision. You know that, for this action, it is necessary for all of the points from the object to. form cones with their bases at the Cornea, and that these leave from the posterior surface of the Lens as cones, each having an axis which falls perpendicularly (or very close to it) on the organ. For it must be supposed that Vision is formed by the feeling [sentiment] of the impression that objects make on the organ when the organ is struck by the species [of light], and that it is only struck feebly by the rays which fall obliquely on it. Now, the place in the Eye where the impression of a large object is made is so small and so narrow that, in a space which seems to be only a point, there must be an infinity of points being received from the object. Thus, a space that (for example) is no larger than the head of a pin, could receive the impression of an object much larger than the Moon, supposing that all of the parts that compose this space on the organ make a field sufficiently capable of directly receiving all of the ends of the cones which have their bases at the Lens. Whereas if this space is rough and uneven, it will only receive the impression of a very small part of the object, so that one could say that it will be seen only imperfectly.
For the same reason, one cannot say that the vessels that are in the Retina are too small to make their interposition prevent the vision of objects, because, even if they were not as large as a hair, this would be more than enough to receive the impression of an infinity of points from the cones by which the representation of an object of a considerable size is formed, particularly if it is far away. For only the evenness of the surface of the organ can make a sufficient number of parts capable of receiving the impression of rays. And it appears that a defect in this evenness, which comes from sickness, old age, or a bad natural disposition, is one of the causes of weakness of vision. In those who cannot distinctly see objects that are far away, one can as much blame this lack of polish of the Retina as the feebleness of visual spirits, or the uncommodious disposition of the Lens. It is easy to conceive that the image of far away things cannot be received except on a very small portion of the organ [and] it is not possible, if the surface is uneven, that it receive, (as it must), a large number of rays in order to have the impression of all the details of this image. On the other hand, all of these details are easily received on a much larger area when the object is closer.
It being thus, it must be remarked that the branches of the vessels that are in the Retina are not capable of causing any inequality in its surface. Because these vessels worm their way into its thickness, they are covered by the upper [final] surface that easily conserves its polish as a result of the disposition of its substance, which is found to be very commodious for producing this evenness. It has a softness and a glary viscosity by which it takes the form of the surface of the vitreous humor that communicates [to it] the polish that all liquid and homogenous bodies ordinarily have in their surfaces. It is made still more even by the medium of the surrounding membranes, which, with a great deal of reason, have been called "vitreous" for their polish and evenness.
It must be agreed that this evenness is lacking in the Choroid and that this defect renders it improper for receiving the impression of species. But it has yet another very considerable [defect] that consists in the nature of its substance. It is completely devoid of the qualities necessary for an organ such as the organ of Vision must be. For this action [of light] is made by a touch incomparably more delicate than those of all our other senses, and so this organ must be provided with a delicateness which renders it permeable to the most subtle spirits and obedient to the lightest impressions. The Retina has all of these qualities in sovereign degree since it is nothing other than the substance of the Brain, the softest and most delicate of all the parts of the body. This substance hardens to form the Optic Nerve, for which this firmness is necessary in order that it pass along such a long path and penetrate the cranial bone, and reassumes its original delicateness, even acquires one still more exquisite, when the Optic Nerve extends over the whole back of the Eye and becomes as if melted or dissolved.
Now, the Choroid has none of these qualities, and if it is a generation of the Pia Mater, which in truth is more delicate and more subtle than all the other parts of the Brain, then it loses this quality in the Eye, where it is incomparably harder, and thicker besides. And besides that, it has a substance and a use that render it, in all truth, incapable of the subtle subtlety that Vision requires. Anatomists have named this membrane "Choroid" because it is filled with a great number of vessels like the membrane that surrounds the Fetus called the Chorion. But it has this [characteristic] in common with many other membranes, and I believe it merits this name better yet by reason of its use, which is like that of the aforesaid membrane that Nature has destined for preparing the blood that the Mother sends for the nourishment of the infant. For dissection has revealed that a great quantity of vessels having many branches are dispersed in the Muscles lying around the globe of the Eye and pierce the Sclera in many places to enter and to spread out in the Choroid. There is great likelihood that the blood, from which the internal parts of the Eye must be nourished, leaves behind [in the Choroid] what it has of coarseness and opacity, for these [internal] parts being admirably clean and transparent, they could only nourish themselves with a substance that, like themselves, was clear and transparent. It is this which makes the Choroid dark and soiled with dirt and with those terrestrial parts of the blood, which as much render it all the more inappropriate to receive the impression from species and the influence of spirits as they give it a great opacity, which is not of the smallest utility for Vision.
The reflections I have made on all these things make me believe that the glary part of the Retina (which is, as I have said, like an effusion of the substance of the Optic Nerve) is the immediate organ of Vision and that the network of filaments which is interspersed there, and which is called the Retina, contributes to this action only by the means of this glary part. This in such a way that they serve more for the distribution of spirits and for other business that this sense has with the Brain, than for the immediate reception of the impression of rays as is believed by some. At least their opinion is repugnant to my system, which establishes [the criterion of] perfectly uniform evenness of surface for the proper organ of vision. Thus, the parts of a membrane that are not continuous are incapable of receiving impressions from all of the points of objects, and that which would be lost in these intervals would cause the loss of a large part of objects. I have explicated my hypothesis as follows.
One can add yet other things to make it obvious that the Choroid cannot be the organ of Vision, e.g., saying that it has no communication with the Optic Nerve; that it is covered at the place where direct vision is formed by another membrane that we call the Pecten [Tapis], which is separate from the Choroid, and which is not always black but is ordinarily tinted and variegated with certain medium and soft colors such as green, blue, gold, silver, mother-of-pearl, etc. From this it appears that color is not a necessary condition for Vision and from this one can draw other consequences as well that are little favorable to the use you would give to the Choroid. I have no doubt that Monsieur Pecquet makes use of these in the Letter he is writing to you.(11) I am content solely with the reasons and facts that I have advanced. For I believe, Monsieur, that if these things are accorded to me, as I believe is reasonable, I would not have much trouble rendering the reason for your Phenomenon, without removing from the Retina the function which is in its possession. For, suppose that evenness of surface is necessary to the organ of Vision: it is not difficult to conceive that the place where the Retina arises from the Optic Nerve will be improper since it cannot have the polish in this place that it has in the rest of the inside of the Eye. For all of the fibers that are distributed in the Retina are branched in this place and do not form the homogeneous substance that is so commodious to the evenness of the surface of which we speak. For this part of the Optic Nerve, which is like a bundle of fibers squeezed in the hole that pierces the Choroid at the place of the Optic Nerve, must be less proper for this evenness than are the extremities of these fibers, effused and dissolved somewhat like the threads of linen are when made into paper, the latter being a very even and well-polished substance if one compares it with the linen itself
One can add,still more, that the place where the Optic Nerve is not yet dilated to mix with the Retina is completely different from the rest of the Retina, whether one conceives that all of the spirits dispersed in the Retina must pass with the utmost impetuosity through this small place, to be collected there; or whether all these fibers, the extremities of which spread the Visual spirits in the dissolved part, are compressed together there. For if the expansion of these fibers, the dilation of the spirits and their tranquillity is proper for vision in all of the rest of the Retina, it is reasonable to conclude that this crushing of fibers near the entrance of the Optic Nerve and the precipitate movement of spirits is not favorable.
Finally, this part of the Retina can also be rendered inappropriate for Vision, as you will appreciate, by the defect of the Choroid,which it pierces. But it does not follow from this that the Choroid is otherwise than one of the organs which contribute something to Vision: to wit, by closing off all avenues of light and preventing the entrance of light by any other part than the pupil. There is reason to believe that the diaphanous substance of the Eyelids, Muscles, Glands of the Eye, and other parts which are between the Choroid and the Socket, could allow some entrance of light from the back, at least at the place where this defect of the Choroid meets it. Also, it seems that in the necessity to pierce the Choroid, in order to allow the Optic Nerve to pass into the Eye, Nature has taken care to crimp this opening as much as possible. It is found that Nature always forms a hole much narrower than is required for the Optic Nerve, which tightens itself in this spot to re-enlarge thereafter in giving rise to the Retina. Now, this hole, by which the Retina is in some manner illuminated, deprives it of the principal disposition that it must have for Vision, that is, to be capable of alteration by the means by which Vision is made. For the Retina being already illuminated from behind is not capable of being illuminated by the Visual Rays from in front, except very feebly. This is analogous to a room which already has a window open and is only illuminated feebly when one opens a second window, if one compares this [second] illumination with that which it received by the opening of the first [window], which found the room absolutely dark and thus caused a very notable change by the introduction of light.
Thus you see, Monsieur, that if the defect of a part of the Choroid
at the place of the Optic Nerve contributes to the loss of vision, this
does not prove that this membrane is anything other than an organ necessary
for the perfection of this action, just as there are other organs, like
the Pupil, the Ciliar Ligament, the Lens, and the other humors of the Eye,
whose dispositions aid vision, but which could not be reputed to be the
principal organ as is the Retina, etc.(12)
I did not undertake a small matter when I engaged myself to defend the rights of the Choroid and I hardly dare promise myself its happy success. Those who do not have an exact knowledge of the anatomy of the Eye and the rules of Optics can understand neither my reasoning nor the facts which I suppose. And Scholars, particularly Members of the Sect [Sectateurs] of the New Philosophy, being predisposed as they are in favor of the Retina, will constantly search for some new difficulty with which to oppose me.
All that I have said or written on this subject up until now has persuaded only a very few people. And novelty, which is ordinarily so well received, has not been favorable to me in this encounter. I am not discouraged, however. I find my case too good to abandon, and although I actually exhausted all that I know of this matter in my second Letter to Monsieur Pecquet, there still remain many very good reasons to oppose to those which you use to attack my opinion. I acknowledge, Monsieur, that most of your objections are very strongly and very ingeniously invented, but I do not find them convincing, and I believe that I am able to resolve them easily and to cleanse you sufficiently of your doubts.
All of the objections that you make can be reduced to three principles.
The first: That the vessels full of blood that are in the Retina prevent Vision if the Choroid is indeed the true organ [of Vision].
The second: That the Choroid is not proper for this use for many reasons of which the principal are that it is rough and uneven; that it is too hard and too thick; that the vessels full of blood, which are spread out there, leave a dirtiness and a blackness that precludes the good reception of impressions from light; and that this membrane has no commerce with the Optic Nerve.
The third: That the Retina is very proper for being the principal organ of Vision and that, supposing this to be true, it is easy to explain the defect of Vision that occurs at the base of the Optic Nerve, by one or the other of two causes which you adduce.
To follow the same order, I will divide my response into three parts.
In the first, I will make it clear that the vessels of the Retina, and their disposition, furnish very strong proofs for establishing my opinion, very far from destroying it.
The second will contain several reasons and experiments to prove that the Choroid is very proper for the use I attribute to it, of which the most considerable are that it is very polished, even and not rough; that it is neither hard nor thick, but supple and delicate -- and of a condition a little like the Pia Mater in the Brain; that the vessels full of blood, by which it is traversed, aid Vision, far from being harmful to it; that the blackness that they leave there, and with which the Choroid is coated and penetrated, is necessary for rendering it sufficiently sensitive to impressions of light; and that it has a perfect communication with the Optic Nerve and with the Brain.
In the third and last, I will try to make it obvious that the Retina is not proper for the use you would attribute to it and that the two causes that you give for the defect of Vision that one observes in my experiment are not in its nature and have no real existence; and that if they have some existence, they would cause the same defect in the other parts of the Retina and would suppress Vision entirely.
I fear, Monsieur, that those who scorn Philosophy have found a subject of raillery in the diversity of our assertions, which are so manifestly opposed. And I cannot myself discover whence a thing of this nature can arise, [which is that] we are able to see so differently. Is it that we have lost exactitude and precision in our observations? Is it that the eyes of men and animals on which we have made them have different dispositions and structures; or perhaps that the love of inventions and opinions to which we are predisposed fascinates our minds and our eyes, precluding us from making reflections that are contrary to our own hypotheses and making us perceive things other than as they are? But whichever of these may be the cause of these contrary sentiments, I am going to work at explaining mine to you and to complete that which I have promised to do.
To resolve our first difficulty, I suppose three things with which I have no doubt you will agree, since they are very well known to you.
The first [observation] is that, when some place on the organ of Vision has received the impression of a luminous or illuminated object, this impression still continues for some moments. One can see this experience when one rotates a burning coal very rapidly, for it appears to be like a circle of fire because the second impression of the light is made before the first is effaced.
The second [observation] is that, when the fibers of the organ of Vision are disturbed by the reception of some rays which reunite there, contiguous fibers where no rays fall are allowed to be disturbed and give a false appearance of light that amplifies the apparent size of luminous bodies. It is for this reason that the flame of a candle at some distance appears, at night, to be much larger than it ought to appear.
My third supposition is that the Eyes are extremely mobile and that that which enables us to see the exact detail of an entire object so quickly is the promptness with which our eyes proceed across all parts of it with the direct Vision, as one perceives when one reads: for while one perceives all of the lines of a page at one time by oblique vision, one can only read by proceeding successively across all the words (and almost all the letters in each word) with the direct vision.(13) From this it happens that the habit our eyes have from this movement precludes us from easily fixing our eyes on a determined point for a considerable time.
These things being agreed, we can now examine your first objection. You say, Monsieur, that the vessels of the Retina prevent Vision if the Choroid is the true organ, and that it would not prevent it on the anterior surface of the Retina. And you believe that this proposition is a convincing argument for destroying my opinion.
But if you mean that the vessels would cause only some inconsiderable defect in Vision, I can use your assertion against you. For I hold that some of these vessels do cause defects in Vision. And because they cannot make this effect in the surface of the Retina, since you have placed them below it according to your hypothesis, it follows that this surface is not the true organ of Vision, as you pretend.
If you mean that these vessels would be a notable prejudice to the Vision, or would suppress it entirely, here are my thoughts on this subject: First I say that these vessels can cause no defect in the vision of the Choroid when one looks at objects with both Eyes, because then they could harm neither the direct Vision nor the oblique Vision. They cannot harm the direct Vision because there are none of these vessels in the place where the Axis of Vision pierces the Retina, nor in a considerable area around it. They also cannot harm the oblique Vision because the rays from the same luminous point do not fall on the same place in each of the Eyes, and it is for the same reason that when one has both Eyes open, one does not perceive the defect in Vision at the base of the Optic Nerve. I say further that the vessels of the Retina, which are close to the Axis of Vision, can cause no sensible defect of vision in a single Eye, for several reasons, of which the most important are: that the vessels are transparent and not at all opaque; that the small threads of blood which flow there are not the thickness of a hair (that is to say, the twenty-fifth part of a ligne); and that since these vessels are situated for the most part on the vitreous humor, they are too far from the Choroid to intercept all of the rays which come from a luminous point, and they allow [in this manner] enough rays to pass to make the smallest objects visible if the objects are sufficiently bright. As regard the vessels that are further from the Axis of Vision, I agree that there are some in which the threads of blood are large enough to cause some defect of vision, particularly at their departure from the base of the Optic Nerve and at the angles of their branchings.
But, since these defects of Vision are so much less considerable than
that which is made at the base of the Optic Nerve (because the size of
this base is seven or eight times as large as the thickness of the largest
threads of blood in these vessels), it follows that it is very difficult
to perceive them; one will be persuaded of this difficulty if one considers
that, before my observation, one did not perceive the defect that is made
on this base. And it is for this reason that I did not speak of these small
defects in my second Letter to Monsieur Pecquet.(14)
One can, however, notice them, and it is a fact that I must establish as
well as the others that I advance. That is to say, that it is necessary
that I explain to you in what manner you may observe all of the facts that
I am going to suppose.
|For this effect take a very fresh Eye on which, before it is removed from the socket, one has marked two lines on the Cornea, one line vertical and the other horizontal, cut at right angles at the center of this Membrane. After having cut the Optic Nerve at the surface of the Choroid, measure the circumference of the Eye with a little strip of paper about a ligne wide. Mark the middle of this band with a black point, set this mark on the center of the Cornea, and, taking again the measure of the circumference of the Eye with another paper according to one of these traced lines, you will mark on the Sclera the point where the extremities of the band meet on the part opposed to the Cornea. Do the same thing with respect to the other line, and you will find almost exactly the point of the Axis of Vision on the exterior surface of the Sclera. Pierce the eye at this spot with a needle to two or three lignes of depth and, having removed the needle, put in its place a small pin about three lignes in length or a small nail with a flat head. Next, cut the eye in half in the manner that I have explained in my second Letter to Monsieur Pecquet, and you will see distinctly that there are no threads of blood in the place where the small nail has pierced the Retina, nor in a rather considerable area around it. In the eyes of cattle this space corresponds to the oblong opening of the Iris and is almost of the same shape and size. You will also see the disposition of the vessels in the Retina somewhat as they are represented in figure 1 on the following page, in which the circle ABCD represents the Retina at the fundus of the Eye; the small circle ae, the base of the Optic Nerve; AEC, BED, the projections of two lines that one is supposed to cut at right angles to the point E representing Sections of planes which would pass through the two lines drawn on the Cornea; E the point where the Axis of Vision pierces the Retina; edc, afb, two of the large vessels of the Retina whose trunks depart almost always from the midst of the base of the nerve; dl, fi, two small branches of these [blood vessels] that are the closest to point E. It is true that these things might not be precisely the same in all sorts of Eyes, and that if the differences are considerable, one would not be permitted to draw the same consequences. It is necessary next to lift the vitreous humor from the Retina and you will notice that the blood of these small vessels is a very lively red. This sufficiently indicates that the Membranes that contain it are diaphanous and transparent; if they were opaque, the blood there would appear livid, as in the veins in other parts of the body. But, in order to be more assured of this transparency, lift some of these filaments with a needle. This is easy to do for they are, for the most part, at the same level as the Retina. Place a small card underneath them, and when they are stuck to it, cut their extremities and look at what will be on the card with a good Microscope. These small, cut vessels will appear to you a little like the figure marked 2, in which the black line FIL represents the thread of blood and all the rest is the thickness of the Membrane which will appear to you very transparent and larger than the thread of blood.|
Consider now the figure of the Eye marked 3, in which AB represents the Lens; CD the opening of the Iris; a Cone of light produced from a single point of a luminous body; a part of the anterior surface of the Retina close to the axis which serves as the base for the small Cone [which is part] of the large [Cone] . Now, CD, the opening of the Iris, is ordinarily 4/3 of a ligne, and is about the same size; is about six lignes 2/3, or 20/3, and is a third of a ligne. But as (20/3) is to (4/3), thus (1/3) is to : then will be about 1/15 of a ligne. But I have supposed that the small threads of blood of the smallest vessels would only have the size of a twenty-fourth part of a ligne. So will be to the thickness of this small thread of blood as twenty-four to fifteen and in consequence, if the cone of light meets a small vessel in the space , a considerable part of this Cone of light will pass around the small thread of blood to the Choroid. And if this cone of light is made by a star, one would not lose it from view, even if one could fix the Eye for a long time on an indivisible point in the Heavens. But, by my third supposition, the eye is too mobile for this effect, and it must happen that, when, by chance, one fixes the Eye at a point and the star is seen feebly, the impression remains on the field of vision immediately preceding and on that which follows when the Eye fixes itself elsewhere a moment later. This would make the star appear as if one had always seen it equally. This is the same as was said of the burning coal in the first supposition. In consequence, it is not possible for one to perceive these defects of Vision, nor would it appear that one loses a rather large star from the View of the oblique Vision when one looks at another star a little to the side, even if the transparent Membrane which holds the blood had a refraction like that of the Retina. But, being as it is of a sulphurous matter (and for this reason its refraction inclines to be a little closer to that of the Lens), it will produce a refraction of rays which falls behind [the vessels] and the rays will make a little focus of light on the Choroid below the small vessel at point , even if it is but a quarter of a ligne away, or even less. For the difference in refraction of these Membranes and the Retina is so small that the rays, which bend toward the extremities of the small vessels, pass close to the small threads of blood. Those who know the rules of Optics easily comprehend this argument, and those who do not, can know the truth of this effect by the following experiment.
It is necessary to have a very slender tube of glass about a ligne thick and to fill it with ink, allowing it to harden inside. After having wiped off the outside, it is necessary to expose the tube to the Sun and to place a small piece of paper underneath and very close to it. Then one will see that the little tube makes a shadow of some size on the paper. But, if one puts the tube and the paper in very clear water and exposes them to the Sun in the same manner, one will see a very small [focus of] light reunited on the paper directly below the tube; and one can determine that the same thing occurs with regard to the vessels of the Retina. From this it is easy to ascertain that, when the Moon is viewed obliquely, one can perceive no defect of Vision caused by these vessels, because, the Moon being much larger than a star, its light must make a focus of considerable size passing through the transparent Membranes of these vessels. The light of this focus, as well as that which passes through these Membranes to the Choroid, disturbs the fibers of neighboring nerves where no light from the Moon falls [directly] on the Choroid (this organ being about 1/16 of a ligne thick when the concavity at the fundus of the eye is a sphere of seven lignes in radius.) Even if the neighboring nerves were not disturbed, one would not be able to see [the defect], because the light (which is not yet reunited when it traverses the anterior surface of the Retina where these vessels are located) occupies a space larger than the sixteenth part of a ligne. For these reasons, one will see no defect in Vision with regard to a large paper that one sees obliquely, or other objects of a considerable size. And in consequence, there is no place on the Choroid at the fundus of the Eye where some Vision is not made. Here is how I have observed the loss of small objects by the interposition of very large threads of blood.
I take a circle of paper a foot in diameter (represented by the circle QRXT in the fourth figure), which I fix to a dark background. I place a small, very white and very bright piece of paper a half an inch in diameter at a distance of about two feet to the right of the larger paper and I stand about ten feet away from these papers until, while fixing my right Eye on the nearest edge of the large circle of paper, I lose the small one from view, and when fixing my Eye on the other, opposing edge of the circle, the small paper disappears as well. It is not necessary that the small paper be at the same height as the center of the large one, but it should actually be about four inches lower. Finally, I increase or decrease these distances, until, by trial and error, I completely lose the small circle while moving my Eye around the circumference of the large paper; yet, by looking a little to the side in the direction of the points Q,R,S,T, I see it again. Then I perceive that toward the spots marked Y and Z, above and below the vertical diameter of the circle, there is a large space of about three inches, and three or four lignes in breadth, where if I fix my Eye, I again lose the small paper. I can attribute this only to the two trunks of the vessels afb, edc, (Figure l) which depart from the base of the Optic Nerve covering a rather large area of the Choroid, and which are close enough to it to cause a defect in Vision in this area. To perceive the defects of Vision that result from the interposition of the large vessels afb, edc, in other places further from the base of the Nerve, I use two or three bands of paper a half an inch wide and a foot long, marked across with many large, black lines. I apply these bands to the same background between the large and small papers, about two feet higher and at a distance of three or four inches from one another in a vertical position. Then, seating myself at the same distance as before and having my head propped firmly, I proceed through the black lines of these papers, and the white intervals, with the same eye, and I almost always meet some point where I lose the view of the small paper when I fix my Eye [on one line]. This happens, as I believe, when the rays fall on the angles of the branches of the large vessels ab, ec, or on their other parts which have sufficient breadth [to cause a defect]. I have made this experiment more than twenty times and I can assure you of having almost always succeeded. But, because vision becomes troubled a little when one fixes the Eye on some point for too long, one often loses the view of a small object which is far to the side: in order to assure myself that the loss of the small paper that I incurred did not proceed from this cause, I closed the Eye for a little while, always holding it in the same position, and opening it suddenly, I would fix it on the point that I had noticed and often the small paper disappeared [again]. Then, fixing my Eye a little higher or a little lower, I would see the circle again. This was sufficient to assure me that some defects of vision are made by the interposition of vessels in the Retina. But this experiment is very difficult [to perform] and I believe that few people will have the patience to do it and to accustom themselves (as is necessary) to fixing the Eye on one determined point for a long period of time. For if one can only fix the Eye there for a moment, then one will believe to see always the small paper, following that which was said above. By these experiments and by these arguments you can perceive that the vessels of the Retina furnish me a very strong proof against your System, since Nature has not affected to place these vessels near the place where direct vision is made, since those vessels that she has placed near this area are very small in width, since they all have transparent membranes, since they are as far away from the Choroid as possible, and since all these precautions are necessary to prevent considerable defects in Vision if the Choroid is the principal organ. It is necessary to believe that this Membrane was destined for this use and that this is a very strong proof which would suffice, even if there were no more convincing reasons.
Most of the facts you pose to support [your argument] that the Choroid is not proper to be the organ of Vision and the consequences that you draw from those with which I can agree, seem to me to have little exactitude. But without stopping to consider them in detail, I will be content to mention here the property that I notice in this Membrane, which you could notice, too, as I [have done], if you observe them with the same method.
After having gently lifted the Retina from the Choroid of an Eye that has been cut in half, whether it is that of a man or a bird, I expose the concavity of this last membrane to some objects terminated by perpendicular lines, such as Bell Towers, Towers, Chimneys, etc. I saw there all the extremities of these objects and all their features represented exactly and without confusion, along with the blue of the air: there is no concave mirror that could have represented them better. This would not happen if the Choroid were rough and uneven as you think. It is true that, if I rub it with a finger, I ruin a black coating or thin film which covers it, which is much more delicate than the epidermis of the skin of the hand, and I am stained by the blackness of this coating, an aqueous and clear wetness that the Retina leaves there. Then there appears on my finger small blackish fragments, mixed with a part of that wetness that clings there, which you call a "blackish, badly mixed mud." But you can draw no consequence from this against the polish and evenness of the Choroid when it is in its natural state, any more than if you had rubbed off the mercury which is behind a mirror: when this attaches to your finger it appears uneven like sand or some large powder; you can conclude only that the surface which touches the glass was very polished and very even before you had touched it. I am surprised that you could doubt the evenness of the Choroid, since the same reasoning you used to prove that the anterior surface of the Retina is polished can be used to prove the same with regard to the Cloroid. The concavity of the Sclera being polished and the convex surface of the vitreous humor being so as well, it is difficult [to believe] that the Retina and the Choroid, which are pressed and squeezed between these two surfaces, would not accommodate themselves to their shapes. One can also perceive that the Choroid is polished by simply looking at it, but this appears better in the eyes of four-footed animals because a large part of this Membrane has a whitish color which makes this more easily discernable. I do not judge whether Vision is made on the first surface of the Choroid that you call the Pecten, or whether this covering serves only as an epidermis; it is believable that the fibers of the Pia Mater extend there as well as to the rest of the Choroid, since the black part and the white part have the same unity of fibers.
After having examined this first surface, I lifted out the entire Membrane and I noticed that, in the eyes of men, it is thin and delicate like a sheet of fine paper; that is to say, a little like the Pia Mater in the Brain. I also noticed that there are many small vessels filled with blood in the part contiguous to the Sclera. But these small vessels interlace so well with the parts of the nerve that it is difficult to distinguish them, and for this reason they are no more able to prevent the impression of light on this Membrane, than the vessels which extend and cover the skin of the hand are able to prevent fire from producing the feeling of Heat in all parts of the hand, or the point of a needle from making its prick felt in any place that it is applied, despite the insensible epidermis that covers the nerves. Nor can the small vessels full of blood or other liquors which are spread there do harm to these feelings. It even sometimes happens that one of the fingers of the hand becomes pale and discolored, and it then lacks as sharp a feeling as the others, as if the blood contributed to feeling by heating the Nerves or by some other property. With regard to the blackness which appears in the Choroid, it is absolutely necessary for an exquisite Vision, as I have proven in my second Letter to Monsieur Pecquet. You know as well as I do that if one exposes a white marble and a black marble to the Sun in Summer, the black becomes much hotter than the white, and that, when one cannot light a white paper in the Sun with a convex glass, one has only to rub it with ink or stain it with the juice of some herb, or something else, to be able to see fire there almost in an instant.
I had planned to show here that the Choroid has more communication with the Optic Nerve and, next, with the Brain than does the Retina, but, because you can see the reasons that I gave in my second Letter to Monsieur Pecquet, I believe that it would be useless to repeat them. It suffices to say that, if by the Optic Nerve you mean its pith, the objection that you make to me that it has no commerce with the Choroid is begging the question, since I hold that this part of the Optic Nerve is insensible to light.
I will not stop, either, to repeat the reasons in the same Letter that
show that the Retina is not proper for being the Organ of Vision. I will
add only that its first surface, being considered as indivisible, is a
Mathematical Entity which can neither produce nor receive any natural effect;
if it is considered as having some thickness, then the small vessels filled
with blood that are encountered there would cause considerable defects
in Vision, because the Cones of light would terminate on them. Besides
this, its softness renders it inappropriate for transmitting the impressions
of light to the Brain, whereas the Choroid is very
well disposed for this effect. This is demonstrated with a long piece of wood or with a long taut cord; the cord and the piece of wood easily transmit an impression that they receive from a blow from one of their ends to the other.(15) This would occur only feebly in a long mass of material similar to the mucosity in the organs of the other senses, which all have much in common with the Choroid.(16) From this [one] should be able to determine that all sensations are made by means of membranes that proceed from the Pia Mater with which the nerves are covered, and that the pith of the Nerve serves only to contain spirits or subtle liquors that serve for movements and for several other purposes.
It only remains now, Monsieur, to speak of the two causes that you adduce for the defect of Vision that occurs at the base of the Nerve.
The first is almost the same as one of those given by Monsieur Pecquet, except that in place of a hoop of small threads that depart from the base of the Nerve, you cover it with a bundle of thick fibers. But this hypothesis is contrary to observation, for these fibers have never been perceived by anyone. I have pulled and pressed many Retinas of many different kinds of animals between my fingers; I have looked at them with excellent microscopes, and I have never been able to remark other than a uniform mucosity, without other filaments than those of the small veins and arteries. And it is in consequence of these observations that I deny the existence of the first cause that you give for the defect of Vision that one notices in my experiment.
I also deny the existence of the second [cause]; that is to say, I hold that no sensible light passes across the Optic Nerve to the Eye from behind. The reason is that light that has made many reflections is weaker than direct light. Now, if I exactly cover my two eyes with my two hands, and I hold them firmly there all the time, I would perceive a darkness as complete in turning toward a very bright object as in turning toward a very dark place. However, the flesh of the hands and eyelids is not very thick and is as transparent as the muscles of the Eye, and as the fibers and coverings of the Optic Nerve. In consequence, the light should pass directly through the hands and through the eyelids with as much facility as through the muscles of the Eye, and then by reflection, through the Optic Nerve. From this I conclude that no sensible light can pass behind [the Eye] to traverse the base of this Nerve.
It is as easy for me to prove that, if these causes were true --that
is to say, if there were an organ of fibers which packed the base of the
Optic Nerve, or if a considerable amount of light passed from behind [the
Eye] -- then these things would suppress the vision on the rest of the
anterior surface of the Retina as well as in the small circle which corresponds
directly to this base. Since this small circular surface is no less polished
than that where the Axis of Vision passes --since it is equally contiguous
with the vitreous humor -- the impression that one receives of the light
should not spread any easier than that which one would receive across this
bundle of fibers, which communicate with the Brain. And the precipitous
movement of the Visual spirits, which you suppose in this place, would
hardly less prevent their tranquility toward the Axis of Vision than towards
the part that is directly exposed to these thick fibers. It is still impossible
to understand how it could be that the light, which passed from behind
by the opening left by the Choroid, would only be able to make a tiny impression
precisely on the part of the surface of the Retina to which it corresponds,
since, upon entering there, the light would extend obliquely to all sides.
One sees this by experiment when one allows light that reflects from some
opposing house to enter into a closed room by a very small hole; if one
puts a blank paper in front of this small hole, two or three feet away,
one will see obscure images from diverse parts of the opposing house on
the parts of paper which are to the side of the hole as well as on that
part of the paper which is directly and precisely opposite to the hole.
One will also notice that the objects that one can distinguish in the room
with a very feeble light will be easily distinguished when one opens the
windows: this entirely destroys your second cause, etc.(17)
1. This sentence was added in 1676. See J. Brons, The Blind Spot of Mariotte (1939), p. 10.
2. "Colors are formed by the modification of the light impulse. Black bodies deaden the light rays and deprive them of their force; white bodies reflect light in all directions without changing anything in the course of action. Other surfaces may impart a rotary motion to the light impulse on reflection, and according to the proportion which exists between translateral and rotary motion, the object appears differently colored." (from Descartes' Dioptrics, Disc. I, AT-VI-8992; Disc. V, AT-VI-117-19, in W. Hoorn, As Images Unwind (1972) p.173.)
3. Note that this is the second time that Pecquet has used this experiment (the first time was on page 3 paragraph 5), but he has drawn two different conclusions from it, the first macroscopic and the second microscopic. Mariotte (page l3 paragraph 1) was rhetorically "confused" by this.
4. This phenomenon is more fully explained by Perrault (page 23 paragraph 2).
5. 1.13 mm or about 1/23 of an inch; Huygens, Oeuvres, Vol. 13 II, p. 787, footnote 3.
6. Houpe à poudrer: the equivalent of a modern powder puff used by the French for powdering their hair or wigs. A good visual analogy for these passages would be an inverted shaving brush.
7. This letter first appeared in 1668 along with Mariotte's first letter in a small book entitled Nouvelle découverte touchant la veüe.
8. Perrault identifies this membrane as the pecten in his letter (page 24, paragraph 2).
9. This letter first appeared in 1671 as Second lettre de M. Mariotte a M. Pecquet pour montrer que la Choroïde est le principale organe de la veüe. A Paris, chez Jean Cusson, in-4o.
10. Perrault was the senior editor of the Mémoires pour servir à l'histoire naturelle des animaux (1671) which was the result of his work with Pecquet and several other members of the Académie Royale des Sciences. "Description anatomique d'un autre lion" (dissected in 1669, or a year after the controversy began) contains this description: "The Retina was very white and very opaque which makes it determined that it would be harmful to the reception of species [of light] if it is true that they pass further. The place where vision is made was ordinarily traversed by a vessel full of blood which also passed within the Optic Nerve, where it made a cavity and seemed to form the pore or conduit by which the Authors have believed that the Optic Nerve is pierced to give passage to the spirits that are carried from the eye, where the species are received, to the Brain" (page 7). The "Description anatomique de cinq gazelles" (pp. 40-46) contains similar observations.
11. "... et que je ne doute point que Monsieur Pecquet ne fasse valoir dans la Lettre qu'il vous écrit." (Mariotte, Oeuvres, p. 522.) This line is important because it indicates that this letter was written before Pecquet's death in 1674, although it was not published until 1676. It also indicates that Pecquet at least started a second letter which was never published (see Oldenburg, Correspondence, Vol. VIII, p. 524 for corroboration).
12. This letter first appeared in 1676 in Receuil de plusieurs traitez de Mathématique de l'Académie Royale des Sciences, "Lettres écrites par MM. Mariotte, Pecquet et Perrault sur le sujet d'une nouvelle découverte touchant la veüe faite par M. Mariotte." A Paris, de l'Imprimerie Royale, in-folio. The revised edition of Mariotte's first letter appears here.
13. Both Kepler (Gregg, The Story of Optometry) and Descartes (Hoorn, As Images Unwind) made a distinction between the central vision (that which is formed on the fundus of the eye and upon which we focus) and the peripheral vision (the out-of-focus images that are formed on the rest of the retina).
14. This is not entirely accurate. See page 18, paragraph 1.
15. These examples were drawn directly from Mariotte's Traitez de la percussion ou chocq des corps, which was published in 1676 (Mariotte, Oeuvres, pp. 1 - 117). Propositions V, VI, and XXVII in Part I and Proposition VIII in Part II seem to have the most relevance to the examples cited here. The concept of an impression carried by a continuum of matter was the underlying basis for his belief that the pia mater, and not the pith of the nerves, carried nervous impulses, and also formed the basis for many of his arguments about what constituted a direct communication with the brain.
16. The context and content of this line make it probable that Mariotte meant Retina rather than Choroid here.
17. This letter first appeared in 1676 along with the letter of Perrault. See the footnote on page 26. All five letters were reprinted in Mariotte's Oeuvres, 1717.