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perception of distance, and the wonderful coördination of muscles necessary for standing and walking, and directing the movements of the eyes. A young ruminant animal in a few minutes after birth will stand and walk, and direct its motions by sight. A bird of wild species, hatched in a cage and kept in a cage until it is fully fledged and its muscles are sufficiently developed, if then thrown into the air, will fly away with ease, although the coördination of many muscles in the act of flying is something so marvelous that it could not be learned in a lifetime of trial, unaided by inherited capacity. Inherited powers are still more marvelous in the case of insects.

Manifestly, then, the wealth of capacities in all directions possessed by the individual is partly inherited and partly acquired by individual experience. In animals the inherited, in man the individually acquired, wealth predominates. But all wealth is acquired. Even that inherited is ancestral experience accumulated and transmitted by the law of heredity. Even instinct is "inherited experience." Thus, then, it is evident that the property of corresponding points of the two retina, and therefore of identical points in space, is partly inherited and partly acquired by individual experience. It is doubtless wholly the result of experience, but not wholly of individual experience.

Consensual Adjustments.-There are therefore two adjustments of the eye in every voluntary act of sight, viz., focal and axial. In the former, each eye is adjusted by the ciliary muscle to make a perfect image on the retina; in the latter, the two eyes are turned by the recti muscles so that their axes shall meet on the point of sight, and the images of the object looked at shall fall on the central spots. The one is an adjustment for

distinct vision, the other for single vision. There is associated with these still a third adjustment, but of far less importance, viz., the adjustment of the pupil. The pupil contracts and expands not only as the light is bright or faint, but also as the object is near or far. These three adjustments take place together and without distinct volition for each-i. e., by the one voluntary act of looking. They are therefore consensual movements, and usually regarded as indissolubly associated. We shall show hereafter that under certain circumstances they may be dissociated.

The two Fundamental Laws.-There are also two great and fundamental laws by which all visual phenomena are explained, viz., the law of direction and the law of corresponding points. The one gives the true position of all points in space, and therefore entirely explains the apparent anomaly of erect vision with inverted retinal images; the other gives coincidence of corresponding points in the two fields of view, and therefore entirely explains the second anomaly of single vision with two retinal images. Both may in fact be called laws of corresponding points. The one asserts the correspondence point for point of retinal rods and cones with external space, with ray-lines connecting and crossing in the nodal point; the other asserts a correspondence point for point of the rods and cones of the two retinæ, and the coincidence of their representatives in the two fields of view. From the one law flow all the phenomena of monocular, from the other all the phenomena of binocular vision.

All the phenomena of binocular vision are explained by the law of corresponding points. But the phenomena are so numerous, so illusory, and so difficult of analysis, that the connection is by no means obvious.

The science of binocular vision consists in tracing this connection, and thus explaining the phenomena. It will be our object, then, to take up all the most important phenomena of binocular vision, and explain them in this way.

CHAPTER II.

SUPERPOSITION OF EXTERNAL IMAGES.

In the movements of one eye, or of the two eyes if they move together equally in the same direction, as in looking to one side or the other, or up or down, objects seem to stand still, and the eyes or the point of sight to sweep over them. But if we move the eyes in opposite directions, as in converging the optic axes strongly and then allowing them to become again parallel, objects, or rather their external images, seem to sweep like trooping shadows across the field of view; or rather, the fields of view themselves seem to rotate, carrying all their images with them, in a direction contrary to the motion of the eye, and therefore (since the two eyes move in contrary directions) in directions contrary to each other. This phenomenon is not very easily observed, because it is best seen by simple convergence of the eyes on a very near point in space, without any object to direct the convergence, or in trying to look at the root of the nose. Divergence of the eyes may be produced by pressing the fingers in their external corners. In this case also the motion of the images is evident.

Evidently, then, by voluntary motion of the eyeballs in opposite directions, and the consequent motion of the

shadowy images in opposite directions, we may (if we observe the images and control the motion of the eyes) cause them, whether they belong to the same object or to different objects, to approach each other and combine successively. Many curious phenomena thus result, which it is necessary to understand before we approach the more complex phenomena, and especially before we can explain the judgments based upon these phe

nomena.

Combination of the Images of Different Objects.-We have seen that the combination of the two external images of the same object produces single vision. But the external images of different objects may also be combined. Under this head there are several cases.

1. Dissimilar Objects.-We have seen that when the two images of an object fall on corresponding points of the two retinæ, they are thrown outward as external images to the same point in space, superposed, and united, and therefore the object is seen single. If, instead of the two images of the same object, the images of two different objects fall upon corresponding points, evidently they also will be thrown to the same place in space and superposed. In this case, however, there being two objects, there will be four retinal images, only two of which will fall on corresponding points, and also four external images, only two of which will be superposed. But we may confine our attention to the superposed images, or else we may cut off the others from view, or prevent them from forming.

Experiment 1.—If the left hand and the right forefinger, or any two dissimilar objects, be held up before the eyes, say 8 to 10 inches apart, and then the eyes be converged until the right eye looks exactly toward the left hand and the left eye toward the right forefinger,

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