left hand, se', will hide the left piece B from the left eye but not from the right, and then make the combination by crossing the eyes, the combined binocular opaque image will be formed as before; but the monocular images will not appear, because there will be

no other retinal image formed except on the central spots. This is represented in Fig. 39. In case we combine beyond the plane of the objects, then a median screen, sc, Fig. 40, extending from the root of the nose n to the table, midway between the objects, will prevent the formation of the monocular images, as shown.

But in these cases, although the union of the two images is perfect, and although we see nothing but an apparently solid opaque object, even yet the illusion is not absolute; partly because the table is doubled and therefore unreal, and partly because the eye is adjusted to the point of sight, whereas the light comes from the

object, which is either nearer as in Fig. 40, or farther off as in Fig. 39, than that point. We will try therefore still another case.

3. Many Similar Objects regularly arranged.-The illusion is most complete when we combine the images of many similar objects regularly arranged over the whole field of view, such as the regular figures of a tessellated pavement or oilcloth, or of a regularly figured carpet of small pattern, or of a papered wall of regular pattern, or the diamond-shaped spaces of a wire grating. In such a case, when by convergence we combine two contiguous figures immediately in front, other contiguous figures all over the plane also combine. In other words, by the motion of the eyes in opposite directions in convergence, the images of the whole plane of the figured surface are slidden by one eye to the left and by the other eye to the right, until combination takes place again over the whole field. When the combination is effected, if we hold the point of sight steady, the combined images of the figures, at first little blurred, become sharp and clear; and then the whole. figured plane comes forward to the point of sight, and appears there as distinctly as a real object, but on a smaller scale in proportion to the less distance. This is represented in Fig. 41, in which the strong line PP represents the plane of the regular figures 1, 2, 3, 4, 5, etc. When contiguous figures, 6 and 7, are united by convergence at the point of sight, and seen there, then all other contiguous figures, 1 and 2, 2 and 3, etc., all over the plane, will be similarly united, and the whole plane with all its figures will advance and be distinctly seen at the distance p'p'. When by stronger convergence alternate figures, 5 and 7, are combined at a nearer point of sight 5 on the plane p" p"-or (which is the

left hand, so', will hide the left piece B from the left eye but not from the right, and then make the combination by crossing the eyes, the combined binocular opaque image will be formed as before; but the monocular images will not appear, because there will be

ature, and yet the whole so apparently seemed to me I could rap my knuckles or pavement. When thus looking at 3.dge, by a slight relaxation of convergence he image and catch it on the next plane, up it to each successive plane, until it falls ai place.

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gures of the pattern are not larger than the tween the optic centers (2 inches), then it also to unite the figures beyond the real plane the plane P'P'. In this case the figures will portionately enlarged, as shown by the diagram. is difficult by this method to make the image the reason for which we shall soon see.

all cases of illusive images the head ought to be steady. If it be moved from side to side while g at such an image, the image will also move from to side in the same direction as the head if the want of sight be nearer than the object, and in the

posite direction if the point of sight be beyond the oject. It is necessary too, in all experiments on com-ination of images, that the interocular line should be exactly parallel with the line joining the objects to be combined; otherwise one image will be higher than the other.

1

Dissociation of Consensual Adjustments. We have said above that when the combination in case 3 (and so also in the other cases) is first obtained, the image of the figures is not distinct, but afterward becomes sharp. The reason is this: The voluntary of the optic axes (axial adjustment) to a nea an the object carries with it, by consensu stment and pupillary contraction for the nce. But since the lenses are adjusted fo

same) when we use the plane p' p' first obtained with all its figures as a real object, and again combine contiguous figures-the whole plane advances to p" p", and is seen as a distinct object with a still smaller pattern of figures. Using the plane thus obtained again as an object, and uniting its contiguous figures, the whole

plane again advances still nearer, and the figures become still smaller at p'" p'". In this manner I have often distinctly seen a regularly figured wall or pavement on six or seven different planes coming nearer and nearer, and becoming smaller and smaller, until the nearest was within 3 inches of the eyes, and the figures