Vision

A painting that we see will, in our heads, always resembles the actual painting.

• This resemblance is not the cause of our feelings for the painting.
• It does not mean that we had a third eye in our brain to perceive it..

Instead, it is the movements of that impression which acts immediately against our soul (so long as it is united to our body) to create such feelings, as instituted by nature.

All the qualities which we see can be reduced to 6 principal ones:

1. light
2. color
3. situation
4. distance
5. size
6. shape

Light

Light and color alone belong properly to the sense of sight.

The force of the movements coming from the small threads of the optic nerves makes the soul have the sense of light.

The quality of these movements gives the soul a sense of color. This is the same way as:

• the movements of our auditory nerves makes the soul hear sounds.
• the movements of the nerves of the tongue makes the soul taste the tastes

Generally, those of the nerves of the entire body, the sensation of some tickling, when they are moderate, and when they are too violent, some pain; without there being any resemblance between the ideas that it conceives and the movements that cause these ideas.

Those who receive some injury in the eye see an infinity of fires and flashes before them, even though they close their eyes or are in a very dark place.

It follows that this sensation cannot be attributed to anything other than the sole force of the blow, which moves the small nerve fibers of the optic nerve, just as a violent light would. This same force, touching the ears, could make us hear some sound. Touching the body in other parts, could make us feel some pain.

But we must particularly consider here how the quantity of light that is seen consists, that is to say, how the force of each of the small nerve fibers of the optic nerve is moved.

For it is not always equal to the light that is in the objects, but it varies according to their distance and the size of the pupil, and also according to the space that the rays, which come from each point of the object, can occupy at the back of the eye.

For example, point X would send more rays into the eye B if the pupil FF were opened up to G.

It sends just as many rays into the eye B, which is near to it and has a narrow pupil, as it does into the eye A, which is much farther away but has a much larger pupil.

Object VXY does not produce more rays from its various points that enter the back of the eye A than enter the back of the eye B.

Rather, these rays do not extend in the back of the eye A as far as they do in the back of the eye B.

This is why they must act with greater force against each of the extremities of the nerve fibers that they touch.

For example, the space HI is quadruple the size of TR. It has 4,000 small nerve fibers of the optic nerve. TR will contain only the extremities of 1,000.

Therefore, each of these nerve fibers will be moved:

• in the back of the eye A, by the thousandth part of the forces that all the rays that enter have, taken together
• in the back of the eye B, by the quarter of the thousandth part only.

We must also consider that one cannot distinguish the parts of bodies that one looks at, except 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 that come from each point of the object are gathered together in roughly the same way in the back of the eye, and that none come from elsewhere to these same points, as has been abundantly explained already; but also on the multitude of the small nerve fibers of the optic nerve that are in the space that occupies the image at the back of the eye.

If the object VXY has 10,000 parts which send rays in 10,000 different ways to the back of the eye RST and therefore makes 10,000 colors visible.

They will not be able to distinguish more than 1,000 of them if there are only 1,000 of these nerve fibers in the space RST. This is because 10 parts of the object act together against each of these fibers.

They can only move it in a single way. And so the space that each of these fibers occupies should be considered as a single point.

The sensation of light is stronger or weaker depending on:

• the object’s distance
• the pupil’s size
• the image painted in the back of the eye is larger or smaller

How the multitude of the small nerve fibers of the optic nerve serves to make the vision distinct.

And this is why a prairie, which will be painted with an infinity of diverse colors, will appear from afar as all white or all blue; and, in general, why all bodies appear less distinctly from afar than from up close; and finally, why the more space an image of the same object occupies at the back of the eye, the more distinctly it can be seen. This will be observed below.

Why prairies, being painted with diverse colors, appear from afar as only one color. Why all bodies appear less distinctly from afar than up close. How the size of the image serves to make the vision more distinct.

For the situation, that is, the side on which each part of the object is placed relative to our body, we do not perceive it otherwise through our eyes than through our hands; and its knowledge does not depend on any image or any action that comes from the object, but only on the situation of the small parts of the brain from which the nerves originate. For this situation, changing slightly whenever the situation of the members where these nerves are inserted changes, is established by Nature to make not only the soul know in what location each part of the body it animates is, relative to all the others; but also to transfer its attention from there to all the places contained within the straight lines that can be imagined to be drawn from the extremity of each of these parts, and prolonged to infinity.

Fig 21

When the blind person turns his hand A towards E, or C towards E, the nerves inserted in this hand cause a certain change in his brain. This allows his soul to know:

• the location A or C
• all the others that are in the straight line AE or CE

This lets him carry his attention as far as the objects B and D and determine their locations without knowing or thinking about those where his two hands are.

Thus, when our eye or our head turns towards some side, our soul is alerted by the change that the nerves inserted into the muscles, which serve for these movements, cause in our brain.

How one knows towards which side an object is located that one is looking at or pointing to without touching it.

Here, as in the eye RST, we must think 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 makes the soul able to know all the places that are in the line RV, or SX, or TY.

This is why objects can be seen in their true situation, notwithstanding that the picture they imprint in the eye has a completely opposite one.

The blind man can feel:

• the object B to the right, through the intervention of his left hand
• the object D to the left, through the intervention of his right hand.

This blind man does not judge that a body is double, even though he touches it with both hands, so when our eyes are both disposed in the way required to direct our attention to the same place, they should not make us see but a single object there, notwithstanding that a picture is formed in each of them.

The vision of distance depends no more than that of the situation on any images sent from the objects. But first on the shape of the eye; for, as we have said, this shape must be a little different to make us see what is close to our eyes, than to make us see what is further away.

As we change it to proportion it to the distance of the objects, we also change a certain part of our brain, in a way that is instituted by nature to make our soul perceive this distance. And this happens to us ordinarily without our reflecting on it; just as when we grasp some body with our hand, we conform it to the size and shape of that body, and feel it through it, without needing to think about its movements.

We know secondly the distance by the relationship between the two eyes. For as our blind man holding the two sticks AE, CE, whose length I suppose he ignores, and knowing only the interval between Maire, p. 59 his two hands A and C, and the size of the angles ACE, CAE, can from there, as by a natural geometry, know where the point E is.

So, when our two eyes RST and rst are turned towards X, the size of the line Ss, and that of the two angles XSs and XsS make us know where the point X is. AT VI, 138 We can also do the same with the help of a single eye by making it change place, as if, holding it turned towards X, we put it first at point S and immediately after at point s, this will be enough to make the size of the line Ss and of the two angles XSs and XsS come together in our imagination, and make us perceive the distance of the point X.

This by an action of thought, which, being only a simple imagination, does not fail to involve a reasoning similar to that which the Surveyors make, when by means of two different stations they measure inaccessible places. We have another way of perceiving distance, namely, by the distinctness or confusion of the figure, and at the same time by the strength or weakness of the light.

As while we stare fixedly at X, the rays coming from the objects 10 and 12 do not come together so exactly towards R and towards T, at the bottom of our eye, as if these objects were at the points V and Y; from which we see that they are further away, or closer to us, than X is. Then from the fact that the light coming from the object 10 towards our eye is stronger than if that object were towards V, we judge it to be closer: and from the fact that the light coming from the object 12 is weaker than if it were towards Y, we judge it to be further away. Finally, when we already imagine elsewhere the size of an object, or its situation, or the distinctness of its figure and colours, or only the strength of the light coming from it, this can serve us, not properly AT VI, 139 to see, but to imagine its distance. As looking from afar at some body that we are accustomed AT VI, 140 to see close up, we judge its distance much better than we would if its size were less known to us. And looking at a mountain exposed to the sun, beyond a forest covered in shadow, it is only the situation of that forest that makes us judge it the closest. And looking at two ships 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 figures, and their colours, and the light they send towards us, to judge which one is the farthest.

if sometimes we force our eyes to look at the sun or some other very bright light, after a short time they retain the impression of it, so that, even if we hold them closed, it seems that one sees various colors which change and pass from one to another, as they grow weaker: for this can only proceed from the fact that the little filaments of the optic nerve, having been moved extraordinarily strong, cannot be stopped as usual; but the restlessness which is still in them after the eyes are closed, being no longer great enough to represent that strong light which caused it, represents colors less vivid; and these colors change while weakening, which shows that their nature consists only in the diversity of the movement, and is not other than I above supposed it.

Finally, this manifests itself in that colors often appear in transparent bodies, where it is certain that there is nothing that can cause them but the various ways in which the rays of light are received therein, as when the rainbow appears in the clouds, and still more clearly when we see its resemblance in 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 small threads of the optic nerve is moved, because it is not always equal to the light. which is in the objects, but it varies according to their distance and the size of the pupil, and also because of the space that the rays which come from each point of the object can occupy at the bottom of the eye .

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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; and that he sends just as many into this eye B, which is close to him and whose pupil is very narrow, as he does in eye A, whose pupil is much larger, but which is in proportion more distant. And even though no more rays enter from the various points of the object VXY, taken all together, into the fundus of eye A than into that of eye B, 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 if, for example, the space HI is quadruple of TR, and if it contains the extremities of four thousand 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 must also consider that 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. And generally that all bodies are seen less distinctly from a distance than from near; and, finally, that 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

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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.

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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.

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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;

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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

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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.

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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.

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.

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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, and therefore touching there two 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. And 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.

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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.

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; and 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.

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.