then evidently the retinal images of these two objects will fall on corresponding points, viz., on the central spots; and their corresponding external images ought to be thrown to the same place and superposed. Such is actually the fact. The phenomena as they actually appear are as follows: As the eyes begin to converge, the images of both objects double homonymously, and we see now four images. As the convergence increases, the double images separate more and more, until the left image (belonging to the left eye) of the forefinger and the right image of the hand (this belongs to the right eye) are brought together and superposed, and the forefinger is seen lying in the palm of the hand. Of course, as already explained, there will be two other images-one of the forefinger to the right, and belonging to the right eye, and one of the hand to the left, and belonging to the left eye. By shutting alternately one eye and then the other, these belongings of the several images may be tested.

Experiment 2.-Or, again, the same combination may take place without convergence of the eyes, thus: Hold up the two forefingers before the eyes a foot or so distant, and a little more than two inches apart (it should be equal to the interocular distance), and against a bright background like a white wall or the sky. Now look at the wall or the sky: the two fingers will both double, making four images; but the two middle images will unite to form what seems to be one finger. There will be therefore apparently three images: the middle one (the combined images) is opaque like an object; the other two, uncombined, are transparent like ordinary double images. In this case, as we are gazing beyond the finger, the double images are heteronymous. It is therefore the right-eye image of the

right finger (the left of its double images) and the lefteye image of the left finger (the right of its double images) which combine in the middle.

These facts and the conditions under which the combination takes place are illustrated by the accompanying diagrams. In Fig. 35 the right eye, R, is directed toward the object B, and the left eye, L, to

In both figures the letters are the same. R and L, the two eyes; A and B, two objects; a'b, Fig. 35, and ab', Fig. 36, combined images; primed letters, left-eye images; c c, central spots of retina; n, the nose; PP, plane of objects; and pp, plane of sight.

ward the object A. The retinal images of these, falling on the central spots c c, are superposed at the point of sight (where the lines of sight intersect) and seen as a'b, while two shadowy images, a and b', are seen to the right and left. Their position in the plane of sight, and as

determined by the law of direction, is given by connecting the points RA and L B. In Fig. 36 the right eye, R, is directed toward the object A, and the left eye, L, toward the object B. The point of sight is therefore beyond, at the meeting of the optic axes or lines of sight. There the combined images, ab', will be seen, while two other uncombined images will be seen at points determined by the law of direction, represented by continuing the lines RB and LA to the plane of sight. It is evident that in this case the two objects A and B must not be farther apart than the optic centers (interocular space); otherwise the lines of sight will not meet in a point of sight, and therefore the two images will not combine. Simple inspection of the diagrams will explain the phenomena, if the reader will bear in mind that capitals represent objects and small letters external images; and further, that the primed small letters represent left-eye images, the strong lines P P the actual plane of the objects, and the dotted lines pp the plane of sight or of the images.

Many persons will not at first succeed in making these experiments, on account of the difficulty which most persons experience in watching double images and controlling the movements of the eyes. To such we would recommend the following method: Let the two objects set up before the eyes in the first experiment be other than parts of the body of the observer-for example, a card and a rod, or two rods. Then, while looking at the table on which the objects lie, hold up the forefinger-or better, a pencil-between the eyes and the objects. The pencil will of course be double. Now, by bringing the pencil nearer or carrying it farther, its double images will separate or close up. Bring

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the pencil into such a position that its double images shall exactly coincide with the centers of the two objects which you desire to combine. If you now look at the pencil, the ocular convergence will be exactly suitable for combining the objects.

In the cases thus far mentioned there is no illusion: the combined images do not produce the appearance of a real object, as in the case of combined images of the same object producing single vision; because, in the first place, the two objects are dissimilar, and therefore the combination is not perfect; and, in the second place, the illusion is destroyed by the existence of the two other uncombined images. We next try—

2. Similar Objects.-If the two objects, the images of which we desire to combine, are exactly similar, then the combined image will be exactly like a natural object. For example:

Experiment 1.-Place two pieces of money of the same kind on the table, being careful that the stamped figures shall be in the same position. Now, looking down upon them, combine as before. Not only will the outlines of the two pieces combine, but the stamped figures in the minutest details, so that the middle combined binocular image will have all the appearance of a real object. This is illustrated by Figs. 37 and 38, in which the position of parts is reversed, because the eyes are supposed to be looking down. In Fig. 37 the two objects (coins), A and B, are combined by crossing the eyes, and the combined or binocular opaque image will be seen at the point of sight as a'b, while monocular shadowy images, a and b', will be seen right and left. In Fig. 38 the combination is made by looking beyond the plane of the coins, and the coins in this case must not be more than an interocular space apart. The com

bined images, like a real opaque object, will be seen at the point of sight ab', and the two shadowy monocular images right and left, as before, only they are now heteronymous.

In this case, though the combination is perfect, yet the illusion is still not complete, because of the presence of the accompanying monocular images; but the formation of these may be prevented by the use of appropriate screens.

Experiment 2.—If in the first experiment with the money, before combining, we hold two cards, sc, sc', Fig. 39, one in either hand and at about half the distance to the table (the best distance is the plane of combination or plane of sight, for then there will be no doubling of the cards), in such position that the card in the right hand, sc, will hide the right piece A from the right eye but not from the left, and the card in the

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