other by a space exactly equal to the interocular space. Now, the nose is no exception to this law. The nose is always seen double and bounding the common field of view on either side.

Experiment 2.--If two similar objects be placed before the eyes in the horizontal plane of sight, and separated by a space exactly equal to the interocular space, and the eyes be directed to a distant point so that their axes are parallel and the two visual lines shall pass through the two objects, then both objects will be doubled, the double images of each being separated by an interocular space; and therefore two of the four images-viz., the right-eye image of the right object, and the left-eye image of the left object will combine to form a single binocular image in the middle; while the right-eye image of the left object will be seen to the left, and the left-eye image of the right object to the right. Thus there will be three images seen-a middle binocular image, and two monocular images, one on each side, that on the right side belonging to the left eye alone, and that on the left to the right eye alone. Now, the eyes themselves are no exception to this law. In binocular vision the eyes themselves seem each to double-two of the images combining to form a binocular eye in the middle (ail cyclopienne), while the other two are beyond the two images of the nose on either side. Each eye seems to itself to occupy a central position, while it sees (or would see if the nose were not in the way) its fellow on the other side of the double images of the nose.

In other words, in binocular vision, when the optic axcs are parallel, as in gazing on a distant object, the whole field of view, with all its objects, including the parts of the face, is shifted by the right eye a half inter

ocular space to the left, and by the left eye a half interocular space to the right, without altering the relative position of parts. It is evident that, by this shifting in opposite directions, the two eyes with their visual lines are brought together in perfect coincidence, so that corresponding points in the two retinæ seem to be perfectly united.

The facts as thus far stated-both the actual condition of things as we know them, and the visual results as they seem to the binocular observer-are represented in the following diagrams. Fig. 80 shows the actual condition of things, and Fig. 81 the visual result, in the first experiment; Fig. 82 the actual condition of things, and Fig. 83 the binocular visual result, in the second experiment. To explain further: In Fig. 80, R and L are the right and left eyes; N, the nose; 4, the object

in the median plane; the dotted lines v v, the direction of the visual lines. Fig. 81 represents the visual results; E being the combined or binocular eye (œil 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 and a' of the object A; while is the position

of the right eye as it would be seen by the left eye, and 7 of the left eye as it would be seen by the right, if the nose were not in the way, and v and v' are the positions of their visual lines if they were visible lines. Fig. 82 represents the actual condition of things when two similar objects A and B are before the eyes in the visual lines vv; and Fig. 83 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 4, Fig. 80, we use a continuous line or rod, as in Fig. 76. 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 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 occu

pied by a real plane, we shall see two parallel planes bounding the binocular field of view on each side, between which we look.

These facts are represented by the diagrams Figs. 84 and 85. In Fig. 84, 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 L 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. 85 represents the visual results. It is seen that the nose n, the rod B, and the finger-point A of fig. 84 are all doubled, as n n', bb', a a' of fig. 85; while the two eyes, R and L, and the two visual lines, v and v, of fig. 84, 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 85.

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