Images That Form On The Back Of The Eye

by Rene Descartes

You therefore see clearly enough that, in order to feel, the soul does not need to contemplate any images which are similar to the things it feels; but that does not prevent it from being true that the objects we look at impress quite perfect ones in the depths of our eyes, as some have already very ingeniously explained by comparing those which appear in a room when having closed it completely, reserved a single hole, and having placed in front of this hole a glass in the form of a lens, one spreads behind it at a certain distance a white cloth on which the light, which comes from objects outside, forms these images; for they say that this chamber represents the eye; this hole, the pupil; this glass, the crystalline humor, or rather all those parts of the eye which cause some refraction; and this linen, the inner skin, which is composed of the ends of the optic nerve.

But you can be even more certain of it, if taking the eye of a freshly dead man, or failing that, that of an ox or some other large animal, you cut dexterously towards the bottom the three skins which envelop it, so that a large part of the humor M[14] which is there remains uncovered, without there being any of it for that reason which spreads; then having covered it with some white body which is so slender that the light passes through it, as, for example, with a piece of paper or the shell of an RST egg, that you put this eye in the hole of ‘a window made on purpose, like Z, so that it has the front BCD facing some place where there are various objects, like VXY, lit by the sun, and the back where the white body RST is, facing inside of the chamber P where you will be, and into which no light must enter except that which can penetrate through this eye, of which you know that all the parts from C to S are transparent.

Because that done, if you look at this white body RST, you will see there, not without admiration and pleasure perhaps, a painting which will very naively represent in perspective all the objects which will be outside towards VXY, at least if you do in so that this eye retains its natural figure, proportionate to the distance from these objects: for, if you press it more or less than reason, this painting will become less distinct from it; and it is to be noticed that it should be pressed a little more and its shape a little longer when the objects are very close than when they are farther away. But I need to explain here at greater length how this painting is formed, because by the same means I will be able to make you understand several things that belong to vision.

Consider therefore first that, from each point of the objects VXY, there enter into this eye as many rays which penetrate to the white body RST as the aperture of the pupil FF can comprehend, and that, according to what has been said here above both the nature of refraction and that of the three humors K, L, M, all those of these rays which come from the same point bend while crossing the three surfaces BCD, 123 and 456, in the way which is required to reassemble approximately to the same point; and it must be remarked that in order that the painting in question here be as perfect as possible, the figures of these three surfaces must be such that all the rays which come from one of the points of the objects are gather exactly at one of the points of the white body RST; as you see here that those from point X assemble at point S, then from which, those which come from point V also assemble approximately at point R, and those from point Y at point T; and conversely that no ray comes towards S except from the point X, nor towards R except from the point V, nor towards T except from the point Y, and so on from the others. Now, this granted, if you remember what was said above about light and colors in general, and white bodies in particular, it will be easy for you to understand that being shut up in the chamber P, and Casting your eyes on the white body RST, you must see the resemblance of the VXY objects there.

For firstly light, that is to say the movement or action of which the sun or some other of the bodies called luminous, pushes a certain very subtle matter, which is found in all transparent bodies, being repelled towards R by the object V, which I suppose, for example, to be red, that is to say to be disposed to make that the small parts of this subtle matter, which have only been pushed in straight lines by the luminous bodies , also move in a circle around their centers, after having met them, and that their two movements have between them the proportion which is required to make the color red felt, it is certain that the action of these two movements having met at the point R a white body, that is to say a body disposed to send it back to any other side without changing it, must from there be reflected towards your eyes by the pores of this body, which I have supposed for this purpose very slender and as if pierced on all sides, and thus make you see the point R red in color.

Then the light being also repelled from the object X, which I suppose yellow, towards S, and from Y, which I suppose blue, towards T, from where it is carried towards your eyes, it must make S appear to you of color yellow, and T colored blue; and thus the three points R, S, T, appearing of the same colors, and keeping between them the same order as the three V, X, Y, have manifestly the resemblance.

The perfection of this painting depends mainly on three things, namely that the apple of the eye having some size, there enter several rays from each point of the object, as here XB14S, XC25S, YD36S, and all as many others as one can imagine between these three come there from the single point X; and that these rays suffer in the eye from such refractions, that those which come from various points are collected at about as many other various points on the white body RST; and finally that, both the small nets EN and the inside of the skin EF, being black in color, and the chamber P, completely closed and dark, there comes from elsewhere only objects VXY no light which disturbs the action of these rays: for, if the pupil were so narrow that only one ray passed from each point of the object to each point of the RST body, it would not have enough force to be reflected from there in chamber P towards your eyes.

The pupil being a little large, if there were no refraction in the eye, the rays which would come from each point of the objects would spread out here and there in all the space RST, so that, for example, the three points VXY would send three rays towards R, which, reflecting from there all together towards your eyes, would make this point R appear to you of an average color between red, yellow and blue, and quite similar to the points S and T, towards which the same points VXY would also each send one of their rays.

It would also happen almost the same, if the refraction which takes place in the eye were greater or less than it should, in proportion to the size of this eye; because, being too great, the rays which would come for example from the point X would assemble before having reached S as towards M; and, on the contrary, being too small, they would assemble only beyond, as towards P, so that they would touch the white body RST in several points, towards which there would also come other rays from the other parts of the object. Finally, if the bodies EN, EF were not black, that is to say disposed to cause the light which gives counter to be damped there, the rays which would come towards them from the white body RST could return from there, those from T towards S and towards R, those from R towards T and towards S, and those from S towards R and towards T, by means of which they would disturb the action of each other; and the same would also make the rays which would come from the chamber P towards RST, if there were any other light in this chamber than that which the objects VXY send there.

But, after having spoken to you of the perfections of this painting, I must also make you consider its defects, the first and principal of which is that, whatever shapes the parts of the eye may have, it is impossible that they cause the rays which come from various points to all assemble at as many other various points, and that the best they can do is only that all those which come from some point, such as from X, assemble at another point, like S, in the middle of the fundus; in which case there can be only a few of those from point V which assemble precisely at point R, or from point Y which assemble precisely at point T; and the others must deviate from it somewhat all around, as I will explain hereafter. And this is the cause that this painting is never so distinct towards its extremities as in the middle, as has been sufficiently remarked by those who have written on optics; for that is why they said that vision is mainly along the straight line which passes through the centers of the crystalline humor and the pupil, such as is the line XKLS here, which they call the axle of vision.

Note that the rays, for example those which come from point V, deviate around point R all the more as the opening of the pupil is larger: and so that, if its size serves to render the colors of this painting more lively and stronger, it prevents on the other hand that its figures are not so distinct, from where comes that it must be only mediocre. Note also that its rays would deviate even more around the point R than they do, if the point V from which they come were much closer to the eye, as towards 10, or much further away, as towards 11 , than is V at the distance of which I suppose the figure of the eye is proportionate; so that they would make the R part of this painting even less distinct than they do. And you will easily hear the demonstrations of all this, when you will have seen below which figures must have transparent bodies, to make the rays which come from a point assemble in some other point, after having crossed them.

As for the other defects of this painting, they consist in the fact that its parts are reversed, that is to say in a position quite contrary to that of the objects, and in that they are appetized and shortened some more, others less, because of the various distance and situation of the things that they represent almost in the same way as in a perspective painting. As you see here clearly that T, which is towards the left side, represents Y, which is towards the right; and that R, which is to the right, represents V, which is to the left; and, moreover, that the figure of the object V must not occupy more space towards R than that of the object 10, which is smaller, but closer; no less than that of object 11, which is larger, but proportionally further away, except insofar as it is a little more distinct; and, finally, that the straight line VXY is represented by the curve RST.

Now, having thus seen this painting in the eye of a dead animal, and having considered the reasons, one cannot doubt that a very similar one is formed in that of a living man, on the skin interior, in the place of which we had substituted the white body RST, and even that it does not form much better there, because its humors, being full of spirits, are more transparent and have more exactly the figure which is required for this purpose. And perhaps also that in the eye of an ox the shape of the pupil, which is not round, prevents this painting from being so perfect there.

Either the images which one makes appear on a white linen in a dark room are formed there all the same and for the same reason as at the back of the eye; even because they are usually much larger there and form there in more ways, one can more conveniently notice various peculiarities of which I wish here to warn you, so that you may experience them, if you do not. you’ve never done before.

First see, then, that if you don’t put any glass in front of the hole you have made in this room, some images will appear on the linen, provided the hole is very narrow, but which will be very confused and imperfect. and which will be all the more so as this hole will be less narrow; and that they will also be all the greater the greater the distance between it and the linen: so that their size must have approximately the same proportion with this distance as the size of the objects which cause them with the distance that is between them and this same hole. As it is obvious that, if ACB [15] is the object, D the hole, and EGF the image, EG is to FD as AB is to CD. Then, having placed a glass in the form of a lens in front of this hole, consider that there is a certain determined distance at which, holding the cloth, the images appear very distinct, and that, as long as it is moved away or when you bring it closer to the glass, they begin to be less so; and that this distance must be measured by the space which is, not between the linen and the hole, but between the linen and the glass: so that, if one puts the glass a little beyond the hole on either side, the linen must also be as much approached or retreated; and that it depends partly on the shape of this glass, and partly also on the distance of the objects: for, leaving the object in the same place, the less the surfaces of the glass are curved, the more the linen must be distant; and, using the same glass, if the objects are very close to it, it is necessary to hold the linen a little further from it than if they are further away from it, and that on this distance depends the size of the images almost in the same way than when there is no glass in front of the hole; and that this hole can be much larger when you put a glass in it than when you leave it completely empty, without the images being much less distinct because of it; and that the larger it is, the more they appear clear and illuminated: so that, if one covers a part of this glass, they will appear much darker than before, but that they will not on that account occupy as much space. space on the laundry; and that the larger and clearer these images are, the more perfectly they are seen: so that, if one could also make an eye whose depth was very great and whose pupil very wide, and that the figures of those of its surfaces which cause some refraction were proportionate to this magnitude,

the images would be formed there all the more visible. And that if, having two or more glasses in the form of lentils, but fairly flat, we join them one against the other, they will have almost the same effect as a single one which would be as much arched or convex as the two together, because the number of surfaces where the refractions take place does not matter much; but that, if we remove these glasses at certain distances from each other, the second will be able to straighten the image that the first will have reversed, and the third will reverse it again, and so on: which are all things whose reasons are very strong. easy to deduce from what I have said; and they will be much more yours, if you have to use a little thought to conceive them, than if you find them better explained here.

Moreover, the images of objects are not only thus formed at the back of the eye, but they also pass beyond to the brain, as you will easily hear, if you think that, for example[16], the rays which come into the eye of object V touch at point R the extremity of one of the small filaments of the optic nerve, which originates from place 7 of the inner surface of the brain 7 8 9; and those of the object X touch at point S the end of another of these threads whose beginning is at point 8; and those of the object Y touch another at the point T, which corresponds to the place of the brain marked 9, and so of the others; and that the light being nothing else than a movement or an action which tends to cause some movement, those of its rays which come from V towards R have the force to move the whole net R 7, and consequently the place brain labeled 7; and those which come from X towards S, to move the whole nerve S 8, and even to move it in a different way than R 7 is moved, because the objects X and V are of two different colors; and likewise those who come from Y move point 9; whence it is manifest that a painting 7 8 9 is again formed, rather similar to the VXY objects, in the interior surface of the brain which looks at its concavities; and from there I could still transport it to a certain little gland which lies about the middle of its concavities, and is properly the seat of common sense. I could even further show you how sometimes it can pass from there through the arteries of a pregnant woman to some determined member of the child she carries in her womb, and form there those marks of envy which cause so much admiration to all learned men.