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in the median plane; the dotted lines v v, the direction of the visual lines. Fig. 95 represents the visual results ; E being the combined or binocular eye (veil cyclopienne); n and n', the two images of the nose belonging to the right and left eyes respectively; V, the combined or binocular visual line, looking between the double images a anl a' of the object A ; while r' is the position

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of the right eye as it would be seen by the left

the left eye, and l of the left eye as it would be seen by the right, if the nose were not in the way, and vand v' are the positions of their visual lines if they were visible lines. Fig. 96 represents the actual condition of things when two similar objects A and B are before the eyes in the visual lines v v; and Fig. 97 is the visual result, in which a' and b are the monocular images, one belonging to the left and the other to the right eye, AB the combined or binocular image, and the other letters representing the same as before.

Experiment 3.-These facts are brought out still more clearly if, instead of an object like A, Fig. 94, we use a continuous line or rod, as in Fig. 90, page 250. We have seen above that, with the optic axes parallel, any object placed in the median line of sight, at whatever distance, is separated into two images an interocular

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space apart. Evidently, therefore, the median line of sight itself is doubled, and becomes two lines, which, resting on the nose on each side, run out parallel to each other indefinitely. Between these two lines the binocular eye (combined eyes) looks out along the combined visual line at a distant object. If the median line be occupied by a real visible line or a rod, we shall see two parallel lines or rods. If the median plane be occupied by a real plane, we shall see two parallel planes bounding the binocular field of view on each side, Letween which we look.

These facts are represented by the diagrams Figs. 98 and 99. In Fig. 98, B represents a rod resting on the root of the nose n, and held in place by the point of the finger A; R and I are the two eyes, and v and v the two visual lines in a parallel position. Such is the actual condition of things. Now Fig. 99 represents the visual results. It is seen that the nose n, the rod B, and the finger-point A of Fig. 98 are all doubled, as n n', 6b', a a' of Fig. 99; while the two eyes, R and L, and the two visual lines, v and v, of Fig. 98, are combined in the middle as the binocular eye E, which looks out along the combined visual line V between the parallel rods b b' of Fig. 99.

As already stated, if instead of a rod we use a plane coincident with the median plane, then the plane is doubled, and we look between the doubled images. This is the case in using the stereoscope. The median plane of the stereoscope is doubled, and between its two images we look out on the combined pictures.

Experiment 4.—An excellent illustration of the fundamental fact, that in binocular vision the two eyes are moved to the middle and combined into a binocular eye, must be familiar to every one who has ever worn spectacles. If the spectacles are properly chosen, so that the distance between the centers of the two glasses is exactly equal to the interocular space, then we see but one glass exactly in the middle, through which the binocular eye seems to look. We would see two other glasses, monocular images, right and left, if these were not hidden by the nose. We do indeed see two others in these positions if we remove the spectacles to such 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. 97 and 99) representing visual results.

If, in place of the reflexion 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. e., if projected on a plain surface. I reproduce as Fig. 100 the diagram already used on page 106, representing rudely the general character of the field of view of the binocular observer. I have introduced the wil cyclopienne and the two monocular images of the eyes; and, in order to make it more comprehensible, I have supposed the observer 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 nn, n' n', is the common or binocular field occupied by the central binocular eye E, sur

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DIAGRAMMATIC OUTLINE OF FIELDS OF VIEW OF Right AND LEFT EYES AND

OF THE COMMON FIELD,

rounded by the single ellipse of the combined spectacleglasses. I have also introduced in dotted outline the left eye 1 and the spectacle-rim & & 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 ohject, 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

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