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in exquisite miniature, and yet the whole so apparently real that it seemed to me I could rap my knuckles against the wall or pavement. When thus looking at the nearest image, by a slight relaxation of convergence we may drop the image and catch it on the next plane, and again drop it to each successive plane, until it falls to its natural place.
If the figures of the pattern are not larger than the distance between the optic centers (24 inches), then it is possible also to unite the figures beyond the real plane -i. e., on the plane P' P'. In this case the figures will be proportionately enlarged, as shown by the diagram. But it is difficult by this method to make the image clear, the reason for which we shall soon see.
In all cases of illusive images the head ought to be held steady. If it be moved from side to side while gazing at such an image, the image will also move from side to side in the same direction as the head if the point of sight be nearer than the object, and in the opposite direction if the point of sight be beyond the object. It is necessary too, in all experiments on combination 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.
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 clear and sharp. The reason is this: The voluntary adjustment of the optic axes (axial adjustment) to a nearer distance than the object carries with it, by consensus, the focal adjustment and pupillary contraction for the same distance. But since the lenses are adjusted for a nearer
distance than the object, the retinal image will be indistinct. The subsequent clearing of the image, therefore, is the result of a dissociation of the axial and focal adjustments. The optic axes are adjusted for the point of sight or distance of the illusive image, and the lenses are adjusted for the distance of the object. Some persons do not find it easy to make this dissociation, and therefore to make the illusive image perfectly clear. To presbyopic persons it is not difficult, but normal eyes will find some, though not insuperable, difficulty.
Now it becomes an interesting question: When the axial and focal adjustments are thus dissociated, with which one does the pupillary contraction ally itself? I answer, it allies itself with the focal adjustment. This may be proved as follows:
Experiment.—While the combination and the formation of the illusive image are taking place, let an assistant standing behind observe the pupil in a small mirror suitably placed in front and a little to one side of one eye. He will see that at first the pupil contracts strongly, associating itself with the axial and focal adjustments to the point of sight; but as soon as the illusive image clears and becomes distinct, he will observe that the pupil has enlarged again.
General Conclusions. It is evident, therefore, that the combination of the similar images of two different objects may produce the same visual effect as the combination of the two images of the same object. In other words, single vision, or ordinary perception of objects, is by combination of two similar images; and it makes no difference whether the two images belong to the same object or to two different but similar objects. This idea must be clearly apprehended and held fast; otherwise all that follows will be unintelligible.
Again, it is evident that two objects may be seen as one, and, contrariwise, one object may be seen as two images. We see then the absolute necessity, in binocular vision, that we should speak of seeing only external images, the signs of objects. They are usually-i. e., under ordinary conditions—the true signs, but often untrue, deceptive, illusory signs.
Thus far we have investigated the case of flat objects, or of figures or colored spaces on a plane. We have shown how the images of these may be combined at pleasure, so as to give the illusory appearance of objects or figures at places and of sizes different from their real places and sizes. We come now to the more complex case of solid objects of three dimensions, and of objects situated at different distances. This brings us to the important subject of the perception of depth of space so far as this is connected with binocularity; or, in other words, to the subject of binocular perspective. We will introduce the subject with some simple experiments.
Experiment 1.—Place one forefinger before the other in the median plane, as in the experiment on page 94. As already seen, when we look at the farther finger, the nearer one is doubled heteronymously; when we look at the nearer finger, the farther one is doubled homonymously. We can not see them both single at the same time. The reason is obvious. If we shut one eye, say the left, we see the fingers as in Fig. 42, I; if we shut the right eye, we see them as in Fig. 42, II. Now these two can not be combined, because they are different. When we combine the images of the farther fingers, a and a', the nearer, 6 and b', will not have come together yet, and will therefore be heteronymously
double, as in Fig. 43, I; when by greater convergence we combine the images 8 and b' of the nearer finger, then the images a and a' of the farther will have crossed. over and become homonymously double, as in Fig. 43, II. As in previous experiments, double images are given in dotted outline, and left-eye images are marked with primed letters, and combined images with capitals.
Now, in this experiment we are distinctly conscious of a greater convergence of the optic axes necessary to combine the double images of the nearer finger, and of a less convergence to combine the double images of the farther. Thus the eyes range back and forth by greater and less convergence, combining the double images of the one and the other, or transferring the point of sight from one to the other; and thus we acquire a distinct perception of distance between the two. It is literally a rapid process of triangulation, the base-line being the interocular distance.
Experiment 2.-We take a rod about a foot long, and hold it in the median plane a little below the horizontal plane passing through the eyes, so that we can see along its upper edge, the nearer end about six or