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WE have already seen, in the history of the stereoscope, that in the binocular vision of objects, each eye sees a different picture of the same object. In order to prove this, we require only to look attentively at our own hand held up before us, and observe how some parts of it disappear upon closing each eye. This experiment proves, at the same time, in opposition to the opinion of Baptista Porta, Tacquet, and others, that we always see two pictures of the same object combined in one. In confirmation of this fact, we have only to push aside one eye, and observe the image which belongs to it separate from the other, and again unite with it when the pressure is removed.

It might have been supposed that an object seen by both eyes would be seen twice as brightly as with one, on the same principle as the light of two candles combined is twice as bright as the light of one. That this is not the case has been long known, and Dr. Jurin has proved by experiments, which we have carefully repeated and found correct, that the brightness of objects seen with two eyes is only 7th part greater than when they are seen with one eye.1 The cause

1 Smith's Opticks, vol. ii., Remarks, p. 107. Harris makes the difference 'oth or 11th; Optics, p. 117.

of this is well known. When both eyes are used, the pupils of each contract so as to admit the proper quantity of light; but the moment we shut the right eye, the pupil of the left dilates to nearly twice its size, to compensate for the loss of light arising from the shutting of the other.1

This beautiful provision to supply the proper quantity of light when we can use only one eye, answers a still more important purpose, which has escaped the notice of optical writers. In binocular vision, as we have just seen, certain parts of objects are seen with both eyes, and certain parts only with one; so that, if the parts seen with both eyes were twice as bright, or even much brighter than the parts seen with one, the object would appear spotted, from the different brightness of its parts. In Fig. 6, for example, (see p. 14,)

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the areas BFI and CGI, the former of which is seen only by the left eye, D, and the latter only by the right eye, E, and the corresponding areas on the other side of the sphere, would be only half as bright as the portion FIGH, seen with both eyes, and the sphere would have a singular appearance. It has long been, and still is, a vexed question among

1 This variation of the pupil is mentioned by Bacon.

philosophers, how we see objects single with two eyes. Baptista Porta, Tacquet, and others, got over the difficulty by denying the fact, and maintaining that we use only one eye, while other philosophers of distinguished eminence have adopted explanations still more groundless. The law of visible direction supplies us with the true explanation.

Let us first suppose that we look with both eyes, R and L, Fig. 7, upon a luminous point, D, which we see single,

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though there is a picture of it on the retina of each eye. In looking at the point D we turn or converge the optical axes dнD, d'H'D, of each eye to the point D, an image of which is formed at d in the right eye R, and at d' in the left eye L. In virtue of the law of visible direction the point D is seen in the direction dD with the eye R, and in the direction d'D with the eye L, these lines being perpendicular to the retina at the points d, d'. The one image of the point D is therefore seen lying upon the other, and consequently seen single. Considering D, then, as a single point of a visible object AB, the two eyes will see the points A and B single by the same process of turning or converg


ing upon them their optical axes, and so quickly does the point of convergence pass backward and forward over the whole object, that it appears single, though in reality only one point of it can be seen single at the same instant. The whole picture of the line AB, as seen with one eye, seems to coincide with the whole picture of it as seen with the other, and to appear single. The same is true of a surface or area, and also of a solid body or a landscape. Only one point of each is seen single; but we do not observe that other points are double or indistinct, because the images of them are upon parts of the retina which do not give distinct vision, owing to their distance from the foramen or point which gives distinct vision. Hence we see the reason why distinct vision is obtained only on one point of the retina. Were it otherwise we should see every other point double when we look fixedly upon one part of an object. If in place of two eyes we had a hundred, capable of converging their optical axes to one point, we should, in virtue of the law of visible direction, see only one object.

The most important advantage which we derive from the use of two eyes is to enable us to see distance, or a third dimension in space. That we have this power has been denied by Dr. Berkeley, and many distinguished philosophers, who maintain that our perception of distance is acquired by experience, by means of the criteria already mentioned. This is undoubtedly true for great distances, but we shall presently see, from the effects of the stereoscope, that the successive convergency of the optic axes upon two points of an object at different distances, exhibits to us the difference of distance when we have no other

possible means of perceiving it. If, for example, we suppose G, D, Fig. 7, to be separate points, or parts of an object, whose distances are GO, DO, then if we converge the optical axes HG, H'G upon G, and next turn them upon D, the points will appear to be situated at G and D at the distance GD from each other, and at the distances OG, OD from the observer, although there is nothing whatever in the appearance of the points, or in the lights and shades of the object, to indicate distance. That this vision of distance is not the result of experience is obvious from the fact that distance is seen as perfectly by children as by adults; and it has been proved by naturalists that animals newly born appreciate distances with the greatest correctness. We shall afterwards see that so infallible is our vision of near distances, that a body whose real distance we can ascertain by placing both our hands upon it, will appear at the greater or less distance at which it is placed by the convergency of the optical axes.

We are now prepared to understand generally, how, in binocular vision, we see the difference between a picture and a statue, and between a real landscape and its representation. When we look at a picture of which every part is nearly at the same distance from the eyes, the point of convergence of the optical axes is nearly at the same distance from the eyes; but when we look at its original, whether it be a living man, a statue, or a landscape, the optical axes are converged in rapid succession upon the nose, the eyes, and the ears, or upon objects in the foreground, the middle and the remote distances in the landscape, and the relative distances of all these points from the eye are instantly perceived. The binocular relief thus

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