the previous visual results, Figs. 88 and 90, it will be seen that the double images b b' approach each other until they unite at the point of sight, and the two images of the rod cross each other at this point, and therefore become again double beyond, but now homonymously. If by still greater convergence we look at a still nearer point C, Fig. 93, then the double images of the median rod, Figs. 87, 89, 91, will cross at the point of sight C, and give the visual result shown in Fig. 94.

Finally, if the point of sight by extreme convergence be brought to the root of the nose, then the double images of the nose n n', Figs. 92, 94, will be brought in contact, and the common or binocular field will be obliterated. In all cases it will be observed that the combined eyes look along the combined visual lines through the point of sight, and onward to infinite distance.

It is evident, then, that in optic convergence, as the two real eyes turn in opposite directions on their optic

centers, the two fields of view turn also on the center of the binocular eye in directions opposite to the real eyes, and therefore to each other.

It will be observed that in speaking of visual phenomena I have used much the same language as other writers on this subject, and used also a somewhat similar mode of representation; only I have substituted eyes in the place of the nose, and put noses in the position of the eyes. I have made median lines cross each other at the point of sight, instead of visual lines, and visual lines combine in the middle as a true median visual line. In other words, I have used the true language of binocular vision. I have expressed what we see, rather than what we know-the language of simple appearance, rather than that mixture of appearance and reality which forms the usual language of writers on this subject.

Second Law. The second law may therefore be stated thus: In turning the eyes in different directions without altering their convergence, objects seem stationary, and the visual lines seem to move and sweep over them; but when we turn the eyes in opposite directions, as in increasing or decreasing their convergence, then the visual lines seem stationary (i. e., we seem to look in the same direction straight forward), and all objects, or rather their images, seem to move in directions contrary to the actual motion of the eyes. The whole fields of view of both eyes seem to rotate about a middle optic center, in a direction contrary to the motion of the corresponding eyes, and therefore to each other. This is plainly seen by voluntarily and strongly converging the eyes on an imaginary very near point, as for example the root of the nose, and at the same time watching the motion of the images of more

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 successively by the two visual lines, are brought from 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, heteronymously. 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 (œil 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 exactThis may be done very easily in the following

ness.

manner:

Experiment. Take a pair of dividers and hold it at arm's length against the sky or a bright cloud, and,

FIG. 95.

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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. 95. 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 almost exactly 24 inches (63.5 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.

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 For this purpose I

find it most convenient to use a small rectangular blackboard about 18 inches long and 10 inches wide, Fig. 96. Mark

FIG. 96.

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 the