distance that the nose no longer conceals them, while we still look through the middle glass at a distant object.

Many other familiar illustrations may be given. If we put our face against a mirror, so that forehead and nose shall touch the glass, and then gaze on vacancy, there will be of course four images of the two eyes in the mirror. Two of these, viz., the right-eye image of the right eye and the left-eye image of the left eye, will unite to form a central binocular eye, an image of our own central binocular eye, and into which our own seems to gaze. The nose will be seen double and on each side of the central eye, and beyond the double images of the nose on either side will be seen monocular images of the eyes. In other words, we actually see exactly what I have expressed in the diagrams (Figs. 83 and 85) representing visual results.

If, in place of the reflection of our own face in a mirror, we make use in this experiment of the face of another person, placing forehead against forehead, nose against nose, and the eyes exactly opposite each other, and gaze on vacancy, the same visual result will follow. Our own central binocular eye looks between our two noses into another central binocular eye, situated also between two noses. Other monocular eyes are seen beyond the noses, right and left.

The fields of view of the two eyes are bordered by the nose, the brows, and the cheeks. Its form therefore varies in different persons. It has no definite limit on the outside. I reproduce as Fig. 86 the diagram already used on page 91, representing rudely the general character of the field of view of the binocular observer. I have introduced the ail cyclopienne and the two monocular images of the eyes; and, in order to make it more comprehensible, I have supposed the ob

server to wear glasses. In this diagram, n n is an outline of the nose, br of the brow, and ch of the cheek of the right-eye field; br', n' n', and ch', the outline of the left-eye field. The middle space where they overlap, bounded on each side by the outline of the nose, n n, n'n', is the common or binocular field occupied by the central binocular eye E, surrounded by the single ellipse

of the combined spectacle-glasses. I have also introduced in dotted outline the left eye l and the spectaclerim 8 8 as they would be seen by the right eye, and the right eye r' and spectacle-rim s's' as they would be seen by the left eye, if the nose were not in the way.

First Law. We are now in position to formulate the first law. I would express it thus: In binocular vision, with the optic axes parallel, as in looking at a distant object, the whole field of view and all objects in the field, including the visible parts of the face, are shifted by the right eye a half interocular space to the left, and by the left eye the same distance to the right, without altering the relative positions of parts; so that the two eyes with their two visual lines seem to unite to form a single middle binocular eye, and a single

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middle visual line, along which the eye seems to look. It follows that any line, rod, or plane in the median line, as also the nose itself, is doubled heteronymously, and becomes two lines, rods, or planes, parallel to each other, and separated by a space exactly equal to the interocular space. Between the two noses, and between the two parallel lines, rods, or planes, the binocular seems to look out along the middle visual line upon the distant object. Of course, by this shifting of the two fields in opposite directions, all objects in the field are similarly doubled.

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Thus in binocular vision the two eyes seem actually to be brought together and superposed, and corresponding points of the two retina to coincide. The two eyes become actually one instrument. And conversely, this apparent combination of two eyes and their visual lines is a necessary consequence of the law of corresponding points. For images on corresponding points are seen single; all objects on the two visual lines must impress corresponding points, viz., the central spots; therefore the visual lines themselves, if they were visible lines, would be seen single. But where could they be seen single except in the middle? Therefore the two visual lines must combine to form a single middle visual line.

We will next give experiments leading up to the second law. For this purpose let us recur to the experiment with the rod represented by Fig. 84. We reproduce this as Fig. 87, in order to compare with it the results of subsequent experiments. As already explained, if the rod B be placed in the median plane with the nearer end resting on the nose-root n, and the farther end held in place by the point of the finger A, the eyes looking at a distant object, as shown in Fig. 87, which represents the actual condition of things, then

between which the binocular eye will look out along the binocular visual line at the distant object, as shown in Fig. 88, which represents the visual result..

Experiment 1.-Now, while we hold the rod in the position represented by Fig. 87, instead of looking at a distant object with eyes parallel, let the eyes be converged on the finger-point F, so that Fig. 89 shall represent the actual condition of things. We will observe that the double images of the rod represented in the visual result, Fig. 88, approach at their farther end, car

rying all objects in the field with them, until they unite at the point of sight F, and we have the visual result represented in Fig. 90.

Experiment 2.-If by greater convergence we next look at some nearer point B on the rod, as in Fig. 91, which represents the actual relation of parts, then Fig. 92 represents the visual result. By comparing this with