Superphysics Superphysics
Part 6d

How the optic nerve makes the vision distinct

by Rene Descartes Icon
18 minutes  • 3642 words

If we force our eyes to look at the sun or a very bright light, after a short time they retain the impression of it.

Even if we close our eyes, we see various colors which change and pass from one to another, as they grow weaker.

This is because the little filaments of the optic nerve are moved extraordinarily strong.

These cannot be stopped as usual.

The restlessness:

  • stays in them after the eyes are closed.
  • is no longer great enough to represent that strong light which caused it, so it represents colors less vivid

These colors change while weakening. This shows that their nature consists only in the diversity of the movement.

Finally, this manifests itself in that

This is also seen in colors which appear in transparent bodies. These can only be caused by the various ways how the rays of light are received.

This is similar to:

  • the rainbow appearing in the clouds
  • a glass which is cut on several faces

But here we must particularly consider in what consists the quantity of the light which is seen, that is to say of the force by which each of

The light moves the small threads of the optic nerve.

The amount of light is not always equal to the light which is in the objects. It varies according to:

  • their distance
  • the size of the pupil
  • the space that the rays which come from each point of the object can occupy at the bottom of the eye

For example, the point X[17] would send more rays into the eye B than it does if the pupil FF were open as far as G.

  • Eye B has a very narrow pupil and is close to X.
  • Eye A has a much larger pupil but is farther from X.

X sends just as many light to both eyes.

Yet because these rays do not extend there that in the space TR, which is smaller than is HI, in which they extend to the fundus of the eye B, they must there act with more force against each of the ends of the optic nerve than they touch it, which is very easy to calculate;

For example, if the space HI is quadruple of TR, and if it contains the extremities of 4,000 of the small filaments of the optic nerve, TR will contain only those of a thousand, and consequently each of these small filaments will be moved in the fundus of the eye A by the thousandth part of the forces which have all the rays which enter it, joined together, and in the fundus of the eye B by the quarter of the thousandth part only.

We can only distinguish the parts of the bodies that we look at insofar as they differ in some way in color, and that the distinct vision of these colors does not depend only on the fact that all the rays which come from each point of the object are gathered together in about as many other various points at the fundus of the eye, and because no others come from elsewhere towards these same points, as well as t has now been amply explained, but also of the multitude of small threads of the optic nerve which are in the space occupied by the image at the back of the eye.

For if, for example, the object VXY is composed of ten thousand parts, which are arranged to send rays towards the fundus of the RST eye in ten thousand different ways, and therefore to cause ten thousand colors to be seen at the same time , they will nevertheless only be able to make the soul distinguish a thousand of them at the most, if we suppose that there are only a thousand of the threads of the optic nerve in the RST space, as much as ten of the parts of the object acting together against each of these nets can only move it in a single way composed of all those in which they act, so that the space occupied by each of these nets must be considered only as a point: and this is why often a meadow which will be painted with an infinity of all different colors will only appear from a distance all white or all blue.

All bodies are seen less distinctly from a distance than from near.

The more space one can make the image of the same object occupy at the back of the eye, the more distinctly it can be seen, which will be very much to be noticed hereafter.

As for the situation, that is to say the side towards which each part of the object is placed with respect to our body, we do not perceive it any other way through our eyes than through that of our hands; and its knowledge does not depend on any image, nor on any action which comes from the object, but only on the situation of the small parts of the brain from which the nerves take their origin;

for this situation, changing ever so slightly each time that of the limbs in which these nerves are inserted changes, is instituted by nature not only to cause the soul to know in what place is each part of the body which it animates with respect for all the others, but also that she can transfer her attention from there to all the places contained in the straight lines that one can imagine being drawn from the extremity of each of these parts and extended to infinity.

As when the blind man, of whom we have already spoken so much above, turns his hand A towards E, or C[18] too


towards E, the nerves inserted in this hand cause a certain change in his brain which enables his soul to know not only the place A or C, but also all the others which are in the straight line AE or CE, so that She can bring her attention to objects B and D, and determine the places where they are, without knowing for that or thinking at all of where her two hands are. And thus, when our eye or our head turns towards some side, our soul is informed of it by the change which the nerves, inserted in the muscles which serve for these movements, cause in our brain.


As here, in the eye RST[19], it must be thought that the situation of the small filament of the optic nerve, which is at the point R, or S, or T, is followed by another certain situation of the part of the brain 7, or 8, or 9, which causes the soul to know all the places which are in the line RV, or SX, or TY; so that you must not find it strange that the objects can be seen in their true situation, notwithstanding that the picture which they imprint on the eye has quite the contrary. As our blind man can feel at the same time the object B[20], which is on the right, through his left hand; and D, who is on the left, through his right hand.


And, as this blind man does not judge that a body is double, although he touches it with his two hands, so, when our eyes are both arranged in the way that is required to bring our attention to the same place, they should make us see there only one object, notwithstanding that a picture is formed in each of them.

The vision of the distance does not depend more than that of the situation of any images sent from the objects, but primarily of the figure of the body of the eye; for, as we have said, this figure must be a little different to make us see what is near to our eyes, than to make us see what is farther from it; and as we change it to proportion it to the distance of objects, we also change certain part of our brain in a way which is instituted by nature to make our soul perceive this distance:

This usually happens to us without our we made reflections there; just as when we squeeze some body with our hand we conform it to the size and shape of this body, and feel it by means of it without our having to think of its movements.

Secondly, we know the distance by the relation that the two eyes have to each other; because, like our blind man, holding the two sticks AE, CE, of which I suppose he does not know the length, and knowing only the interval which is between his two hands A and C, and the size of the angles ACE, CAE, can from there, as by a natural geometry, to know where is the point E, thus, when our two eyes RST[21] and rst are turned towards X, the size of the line S and that of the two angles XSs and XsS let us know where is the point X. We can also do the same with the aid of one eye alone by making it change its place, as if, holding it turned towards X, we put it first at the point S and then immediately at the point s, this will suffice to cause the magnitude of the line Ss and of the two angles XSs and XsS to be found together in our imagination and make us perceive the distance from the point X;


and this by an action of thought which, being only a very simple imagination, does not fail to envelop in itself a reasoning very similar to that made by surveyors when, by means of two different stations, they measure inaccessible places. We have yet another way of perceiving distance, namely, by the distinction or confusion of figure, and together by the strength or debility of light. As, while we gaze fixedly towards X[22] the rays which come from the objects 10 and 12 do not assemble so exactly towards R and towards T at the back of our eye, as if these objects were at the points V and Y, from where


we see that they are further away or closer to us than is X.

Then, because the light which comes from the object 10 towards our eye is stronger than if this object were towards V, we judge it to be nearer; and because that which comes from object 12 is weaker than if it were towards Y, we judge it to be further away.

Finally, when we already imagine the size of an object, or its situation, or the distinction of its shape and its colors, or only the strength of the light which comes from it, this can serve us not properly to see, but to imagine its distance.

As looking from afar at some body that we are accustomed to seeing up close, we judge its distance much better than we would if the size were less known to us;

looking at a mountain exposed to the sun beyond a forest covered with shade, it is only the situation of this forest that makes us judge it nearest; and looking at two vessels on the sea, one of which is smaller than the other, but closer in proportion, so that they appear equal, we will be able, by the difference in their shapes and their colors, and of the light they send to us, to judge which will be the farthest.


For the rest, for the way in which we see the size and shape of objects, I do not need to say anything in particular, especially since it is entirely included in that of which we see the distance and the situation. of their parts, namely, their size is estimated by the knowledge or the opinion that one has of their distance, compared with the size of the images which they print on the back of the eye, and not absolutely by the size of these images, as is quite clear that, although they are, for example, a hundred times larger when the objects are very close to us than when they are ten times further away, they are not we do not therefore see them a hundred times larger, but almost equal, at least if their distance does not deceive us. And it is also manifest that the figure is judged by the knowledge or opinion that one has of the situation of the various parts of the objects, and not by the resemblance of the pictures which are in the eye; for these paintings usually contain only ovals and lozenges when they show us circles and squares.

But, so that you can have no doubt that the vision takes place as I have explained it, I want you here again to consider the reasons why it sometimes happens that it deceives us: firstly, because it is the soul which sees, and not the eye, and which it sees immediately only through the agency of the brain, hence it is that the frantic and those who sleep often see or think they see various objects which are not for that before their eyes, namely when some vapours, stirring their brains, arrange those of its parts which are accustomed to serve for vision in the same way as these objects would be if they were present.

Finger

Then, because the impressions which come from without pass towards the common sense through the intermediary of the nerves, if the situation of these nerves is constrained by some extraordinary cause, it can make objects in other places appear that they are not. As if the eye rst[23], being disposed of itself to look towards X, is constrained by the finger N to turn towards M, the parts of the brain from which its nerves come will not be quite disposed. in the same way as if it were his muscles that turned him towards M, nor also in the same way as if he were really looking towards X, but in a way between these two, namely, as if he were looking towards Y ; and thus the object M will appear where Y is through this eye, and Y where X is, and X where V is; and these objects also appearing at the same time in their true places through the intervention of the other eye RST, they will seem double.


In the same way that, touching the small ball G[24] with the two fingers A and D crossed one over the other, one thinks of touching two of them, because, while these fingers hold each other thus crossed, the muscles of each of them tend to separate them, A towards C and D towards F, by means of which the parts of the brain, whence come the nerves which are inserted into these muscles, are arranged in the way which is required to make them appear to be A to B and D to E.

Therefore, touching there 2 different balls H and I places to which we must look to smell them, when they happen to come from elsewhere, we can easily be deceived there; like those whose eyes are infected with jaundice, or who look through a yellow glass, or who are shut up in a room where no light enters except through such glasses, attribute this color to all bodies that they are looking at.

The one who is in the dark room, which I described earlier, attributes to the white body RST[25] the colors of the objects VXY, because it is only towards it that he raises his sight.

The eyes A, B, C, D, E, F see the objects T, V, X, Y, Z, U, through the glasses N, O, P, and in the mirrors Q, R, S.

Eyes

These judge them to be at the points G, H, I, K, L, M, and V, Z be smaller, and X, U larger than they are: or also X, U smaller and with that reversed, namely, when they are a little far from the eyes C, F, d as much as these glasses and these mirrors divert the rays which come from these objects in such a way that these eyes can see them distinctly only by arranging themselves as they should be to look towards the points G, H, I, K, L , M, as will easily be known to those who will take the trouble to examine it.

They will see, by the same means, how much the ancients were mistaken in their catoptrics, when they wanted to determine the place of images in hollow and convex mirrors.

All the means available to know the distance are very uncertain; for, as for the shape of the eye, it hardly varies any more sensibly when the object is more than four or five feet away from it; and even it varies so little when it is closer, that no very precise knowledge can be drawn from it.

For the angles comprised between the lines drawn from the two eyes to each other and from there to the object, or from two stations of the same object, they also hardly vary any more when we look even slightly far ; then from which our common sense even does not seem to be able to receive in itself the idea of a greater distance than about a hundred or two hundred feet, as it can be verified from what the moon and the sun , which are among the most distant bodies that we can see, and whose diameters at their distance are about one to a hundred, usually appear to us only one or two feet in diameter at the most. , notwithstanding that we know enough, by reason, that they are extremely large and extremely distant;

for that does not happen to us for lack of being able to conceive them greater than we do, seeing that we conceive many towers and mountains much greater, but because, being unable to conceive them more distant than 100-200 feet, he it follows from this that their diameter must appear to us only one or two feet: in what way the situation also helps to deceive us, because ordinarily these stars seem smaller when they are very high towards the south than when, rising or lying down, there are various objects between them and our eyes which make us better notice their distance; and astronomers prove enough, by measuring them with their instruments, that what they appear to be larger one time than the other is not because they see themselves from a greater angle, but because they consider themselves more distant, whence it follows that the axiom of old optics, which says that the apparent size of objects is proportioned to that of the angle of vision, is not always true. We are also mistaken in that white or luminous bodies, and generally all those which have much force to move the sense of sight, always appear somewhat nearer and larger than they would if they had less. : now the reason which makes them appear nearer is that the movement with which the eye narrows to avoid the force of their light is so joined with that which disposes the whole eye to see near objects distinctly, and by which one judges of their distance, which one can hardly do without also doing a little of the other, in the same way that one cannot entirely close the first two fingers of the hand without the third also curve somewhat as if to close with them.

These white or luminous bodies appear larger because:

  • the estimate which one makes of their size depends on that of their distance
  • their images are impressed larger in the fundus of the eye

The ends of the threads of the optic nerve which cover it are very small but have some thickness.

Each of them can be touched in one of its parts by an object, and in others by others.

Each can be moved in one way each time. When the least of its parts is touched by some very brilliant object, and the others by others which are less so, it follows entirely the movement of that which is most brilliant, and represents its image without representing that of the others.

Balls

As if the ends of these small nets are 1, 2, 3[27], and that the rays which come, for example, to trace the image of a star on the bottom of the eye extend there on that which is marked 1 and somewhat beyond all around on the extremities of the six others marked 2, on which I suppose that no other rays come, except very weak ones, from the parts of the sky near to this star, its image will extend in all the space occupied by these six marked 2, and even perhaps in all that occupied by the twelve marked 3, if the force of the movement is so great that it is also communicated to them .

This is why the large stars appear rather small because of their extreme distance.

A square tower, seen from a distance, appears round. All the bodies which trace only very small images in the eye cannot trace there the shapes of their angles. Finally, as regards judging distance by size, or shape, or color, or light, perspective pictures show us how easily it is to be mistaken; for often, because the things painted therein are smaller than we imagine they ought to be, and their lineaments more confused, and their colors browner or fainter, they appear to us more distant than they do. are not.

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