distant objects. The whole field of view of the right eye, carrying all its images with it, seems to rotate to the right, and of the left eye to the left-i. e., homonymously. The images of all objects, as they are swept Euccessively by the two visual lines, are brought fron opposite directions to the front and superposed. As we relax the convergence, and the eyes move back to a parallel condition, the two fields with their images are seen to rotate in the other direction—i. e, heterony. mously. If we could turn the eyes outward, the two fields and their images would continue to rotate heteronymously. This, which we can not do by voluntary effort of the ocular muscles, may be done by pressing the fingers in the external corners of the two eyes. By pressing in the internal corners, on the contrary, the eyes are made to converge, and homonymous rotation of the fields of view is produced.

Or the law may be more briefly formulated thus: In convergence and divergence of the eyes, the two fields of view rotate in opposite directions, homonymously in the former case and heteronymously in the latter, about the optic center of the binocular eye (œi cyclopienne), while the middle or binocular visual line maintains always its position in the median plane.

Thus, then, there are two apparent movements of the visual fields accomplished in binocular vision. First, there is a shifting of each field heteronymously a half interocular space. This is involuntary and habitual, and would of itself double all objects heteronymously, separating their images exactly an interocular space. Second, in convergence, there is a rotation of each field about the optic center of the oil cyclopienne (or about an axis passing through that center and normal to the visual plane), homonymously. The necessary conse

quences of these movements are: (a) that the images of an object at the point of sight are superposed and the object is seen single, while objects on this side of the point of sight are doubled heteronymously, and those beyond the point of sight homonymously; (b) that all objects (different objects) lying in the direction of the two visual lines, whether nearer than or beyond the point of sight, have their images (one of each) brought to the front and superposed; so that the two visual lines are under all circumstances brought together and combined to form a single binocular visual line, passing from the middle binocular eye through the point of sight and onward to infinity.

In all the experiments which follow on this subject it is necessary to get the interocular space with exactness. This may be done very easily in the following

manner:

FIG. 109.

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Experiment.-Take a pair of dividers and hold it at arm's length against the sky or a bright cloud, and, while gazing steadily at the sky or cloud, separate the points until two of the four double images of the points shall unite perfectly, as in Fig. 109. The distance between the points of the dividers, equal to a-a', or b-b', or c-c', is exactly the interocular distance-i. e., the distance between the central points of the central spots of the two retinæ. The only difficulty in the way of perfect exactness in this experiment is the want of fine definition of the points when the eyes are adjusted for distant vision. This may be obviated by using slightly convex spectacles. The accuracy of the determination may be

verified thus: Measure the distance just determined accurately on a card, and pierce the card at the two points with small pin-holes. Now place the card against the forehead and nose, with the holes exactly in front of the two eyes, and gaze through them at a distant horizon or cloud. If the measurement is exact, the two pin-holes will appear as one; their coincidence will be perfect. As thus determined, I find my interocular space exactly 2:44 inches (62 mm.). It will be seen that this method is founded upon the opposite shifting of the two fields of view half an interocular space each, spoken of in the first law. The two pinholes are seen as one exactly in the middle, which is looked through by the oil cyclopienne; and this is therefore one of the very best illustrations of such shifting of the two eyes and their visual lines to the middle.

FIG. 110.

We will now give some additional experiments illustrating and enforcing these two laws, and showing the absolute necessity of using this new mode of diagrammatic representation in all cases in which binocular perspective is involved. For this purpose I find it most convenient to use a small rectangular blackboard about 18 inches long and 10 inches wide, Fig. 110. Mark two points R and L at one end, with a space between exactly equal to the interocular space, and in the middle between these points make a notch n in the edge of

Such a

the board to fit over the bridge of the nose. board is admirably fitted for all experiments on binocular perspective.

Experiment 1.-Draw a line through the middle of the board from the notch n, Fig. 110. This will be the visible representative of the median line; and as the median line is used in all the experiments, this may le made permanent. On this line place two pins at A and B. Draw also from the points L and R dotted lines

parallel to the median line and to each other, as the visible representatives of the visual lines when the optic axes are parallel, as when looking at a distant object. Now fit the plane over the bridge of the nose, and place it in a horizontal position a little below the primary plane of vision, say half an inch or an inch, so that the whole surface is distinctly seen, and then look

beyond at a distant object. Leaving out the board in the representations, the actual position of the lines is shown in Fig. 111 and the visual result in Fig. 112. Remembering that in all our figures capitals represent combined or binocular images, simple italics right-eye images, and primed italics left-eye images, it will be seen that the whole board, with all the lines and objects on it and the parts of the face, has been shifted left and right by the two eyes, so that the nose and the median line are seen as two noses and two parallel lines with their pins, separated by a space exactly equal to the interocular space, and the two visual lines are brought together and united in the middle to form a common visual line V, as if coming from a single binocular eye E. If two small circles be drawn or a pin be set at the end of the dotted visual lines in Fig. 111, these will be united in the result Fig. 112, at the end of the combined visual line V. There will also of course be seen to the extreme right and left monocular images of the dotted representatives of the visual lines, and of the circles or pins at their farther end. I have connected by vincula the images of the whole drawing, the primed vinculum being the image of the left eye, the other of the right.

Experiment 2.-If we now erase the parallel visual lines v v on the board, and draw them convergent on the pin A, so that Fig. 113 shall represent the actual condition, and then adjust the board again to the nose and look at the pin A, the visual result, or what we shall see, is given in Fig. 114. By comparing this result with the actual condition of things-i. e., by comparing Fig. 114 with Fig. 113—it would seem as if the whole drawing on the board, including the eyes and nose, had been turned about the point of sight A by the two eyes in