CHAPTER XIV.

CAN SIGHT GIVE US THE IDEA OF A SOLID?

78. It has been asserted that sight can not give us the idea of size or of a solid, but that this can be obtained only by the help of touch. I believe the contrary may be proved with convincing certainty.

What is a solid? It is the union of three dimensions. If sight can give us the idea of surfaces which consist of two, why not also of solids which consist of three dimensions? This one reflection is enough to show that it has been denied without reason; but I shall not stop with this, but shall prove, by the most rigorous observation, and the analysis of its phenomena, that sight can give us the idea of a solid.

79. I willingly agree, that if we suppose a man deprived of all the other senses, to have his eyes immovably fixed on an immovable object, he would never be able to distinguish between what is solid and what is merely perspective in the object; or, in other words, that all the objects permanently painted on the retina will appear to be projected on a plane. The reason of this is founded on the very laws of the organ of this sense, and of the transmission of its impressions to the brain.

The soul refers the sensation to the extremity of the visual ray; and since in the present instance it has been unable to make any comparisons, it can have no motive for placing these extremities at unequal distances, which constitutes the third dimension.

In order to understand this better, let us suppose the object to be a cube placed so that three of its sides are seen. It is evident that although the three surfaces are equal, they will not appear so to the eye, because their respective positions do not permit them to send their rays equally to it. But as the soul has not had occasion to compare this sensation with any other, it can not calculate the difference produced by the different positions and distances, but must refer all the points to the same plane, regarding the sides of the cube as unequal; though, in reality, they are not so.

Sight, in this case, presents the whole object on a perspective plane; and as it could have no means of calculating the distance of the object from the eye, it would probably believe it joined to the eye, or, more strictly speaking, the sensation would represent only a simple phenomenon, the relations and cause of which we could not explain.

80. It is likewise probable, that if, while the eye remained fixed, we could open and shut the lids, we might form the idea that the object seen was outside of us; so that by this motion alone, we should obtain a point of comparison, by the succession of the alternate disappearance and reproduction of the sensation of the object by the interposition or non-interposition of an obstacle. Then the idea of a greater or less distance would arise, and as this would be in the direction perpendicular to the plane of the object, we should already have the idea of a solid.

Fortunately, nature has been more beneficent to us, and we are not obliged to limit ourselves to a supposition which thus curtails our means of acquiring ideas. Still it will not be useless to have examined the phenomenon on this supposition; for, from this examination, we shall gain light to understand what I propose to demonstrate.

81. In order that sight may originate the idea of a solid, it requires motion. This motion is an indispensable condition, though it may be either in the object or in the eye itself.

Let us suppose an immovable eye, and see how by the motion of the objects, the sensation of sight may present or produce the idea of a solid. The only difficulty is to show how it can add to the two dimensions which constitute the plane, the third which forms the solid.

Let a fixed eye be directed to a point where there is a right rectangular parallelopipedon B, so placed that its two bases are wholly concealed, and let the right line drawn from the centre of the eye to the edge of the parallelopipedon divide the plane angle into two equal parts. Let us also suppose the sides of the parallelopipedon to be of different colors,--white, green, red, and black. In this case, the eye sees the two planes as one, and the edge appears as a right line separating the two parts of the same plane which differ only in color. It is impossible for it to conceive the inclination of the two planes, because as it refers the object to the extremity of the visual ray, and has not been able to compare the varieties which result from difference of position and distance, and from the manner in which the object receives the light, it can only distinguish the different parts of the same plane.

It is well known that perspective can perfectly imitate a solid. For, if instead of the solid B, we suppose two planes exactly representing the two sides seen, the sensation will be the same, the illusion complete. Therefore, there are two distinct ways of producing the same sensation; and consequently, unless there has been a previous comparison, there is no means of distinguishing them apart; but the idea which would naturally result would be the most simple; that is, the idea of a plane.

82. If we suppose the parallelopipedon B to revolve on a vertical axis, it will present the four planes successively to the eye, and they will appear greater or less according to their inclination to the visual ray, the surface of the plane reaching its maximum when perpendicular to the ray, and its minimum when parallel to it.

The succession and variety of the sensations will immediately produce the idea of motion; for the same planes of the parallelopipedon are seen in different positions. The uniform manner in which these planes succeed one another, will also suggest the idea that the green which appears a few moments after the black, is the same which was seen a few moments before; and so of the other colors. Also, as one is constantly hiding behind the other, this naturally gives rise to the idea of extension in the direction, or continuation of the visual ray; and this is sufficient to produce the idea of size or of a solid. When we see a plane we have the two dimensions which constitute a surface; to form the idea of a solid, we need only the idea of one dimension more; this can not be found in the same plane, but is produced by the motion of the parallelopipedon.

83. This motion which we have supposed to be around a vertical axis, we may equally suppose to take place around a horizontal axis. We shall then see in succession the two opposite sides, and the bases of the parallelopipedon with different appearances, according to their various positions; or, in other words, according to the angle formed by the planes and the visual ray. These appearances will help more and more to form the idea of the third dimension, which is not to be found in the primitive plane, and consequently to supply what was wanted to constitute the idea of a solid.

84. Just as we have supposed the eye fixed and the object movable, we may suppose the eye in motion and the object immovable. The result is the same; for, it is evident that if the eye should move around the parallelopipedon, now vertically, and now horizontally, it would experience the same sensations as when it was quiet, and the parallelopipedon moved. Thus, although we suppose the subject wholly deprived of the sense of touch, so as to be unable to perceive its own motion, it can still form the idea of solid by the impressions of sight alone. True, it could distinguish which moved, the eye or the object, but this does not interfere with the formation of the idea composed of the three dimensions.