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In every such experiment the consciousness is quickly and completely aroused, and the double images are speedily reunited, though not so speedily but that the result is unmistakable. But, lest some may regard the speedy union of the images as an objection to this experiment, we will take another.
Experiment 2.-While lying abed in the morning, if one gazes on vacancy, objects near at hand (say the bedpost) are doubled heteronymously, the images being 2 inches apart. If, while thus gazing and observing the heteronymous images, one should be overtaken by drowsiness and consequent loss of control over the ocular muscles, he will see that the already heteronymous images separate more and more. Now, if this were due to convergence, the heteronymous images would approach, unite, cross over, and become homony
It is certain, then, that in myself, in extreme drowsiness, when control over the ocular muscles is lost, and therefore presumably in sleep, the eyes diverge. I have also satisfied myself that my case is not exceptional in this respect, for my results have been verified by several other persons. I think, therefore, I may assume it as a general law.
Double vision is also a well-known phenomenon of extreme intoxication. The unnatural appearance of the eyes in such cases is due to want of parallelism of the optic axes. I have on several occasions examined the eyes of those in this sad condition, and have always found the axes divergent. This seems to arise from partial paralysis of the ocular muscles.
If we examine the eye-sockets of a human skull, we find that their axes diverge about 25°-30°. This is about the extreme divergence of the optic axes in
drowsiness. It is probable, therefore, that in a state of perfect relaxation or paralysis of the ocular muscles the optic axes coincide with the axes of the conical eyesockets, and that it requires some degree of muscular contraction to bring the optic axes to a state of parallelism, and still more to one of convergence, as in every voluntary act of sight. In the human eye, therefore, and also in that of the highest animals, there are three conditions of the optic axes: first, convergence, as when we look at a near object; second, parallelism, as when we look at a distant object or gaze on vacancy; third, divergence, when we lose control over the ocular muscles, as in drowsiness, in drunkenness, in sleep, and in death. The first requires a distinct voluntary contraction of the ocular muscles; in the second there is no voluntary action, but only that involuntary tonic contraction characteristic of the healthy waking state; in the third the relaxation is complete. The first is the active state of the eye, the second the waking passive state, the third the absolutely passive state.
2. Other Modes of producing Divergence.-But the divergence of the optic axes may be effected in other ways. In most normal eyes the passive state is one of parallelism. It is easy therefore to double homonymously the images of an object at any distance by convergence, but most persons would find it impossible voluntarily to double the images of a very distant object, as for example a star, heteronymously-i. e., by divergence. Yet under certain conditions a slight divergence is possible. For example, I find I can (and I believe most persons can) combine with the naked eyes and with natural perspective (i. e., beyond the plane of the card) stereoscopic pictures in which identical points are farther apart than the interocular distance. I can
not always succeed, being able to do so only when my mind is in an exceptionally passive state.
Experiment 3.-I take now a skeleton stereoscopic diagram, identical points in the background of which are separated by a space greater by an eighth of an inch than my interocular space. By holding it at arm's length so as to make the divergence as small as possible, I succeed in combining. After the combination is stable, I can bring the card nearer and nearer until it is within 5 inches of my eyes, and yet the combinaation is retained. But this corresponds to a divergence of only 110.
Experiment 4.-But by mechanical force we may make the eyes diverge 40° or 50°. This is done by pressure in the external corner of the eye. By thrusting a finger of each hand into the external corners of the eyes I can make the two images of an object directly in front separate 50°, or the images of two objects situated 25° to the right and left of the median line, and therefore 50° apart from each other, come to the front and unite.
The following diagrams represent and explain the visual phenomena in divergence of the optic axes.
In Fig. 141, which represents the actual relation of parts, m is the median line; vv, the visual lines or optic axes produced; A, an object on the median line; bb, two similar objects in the direction of the diverging visual lines; and rr, ray-lines from the object A. Fig. 142 shows the visual result if the lines in Fig. 141 were visible lines drawn on the plane described on page 266. It will be seen that by heteronymous shifting and then heteronymous rotation the whole diagram represented by Fig. 141 has been carried and rotated by the right eye to the position of the lines connected by the unprimed vinculum, and by the left eye to the position
of the lines connected by the primed vinculum. By this means the two visual lines v v are brought together and combined as the common visual line V, and two of the images of the objects b b are brought together and superposed at B; the median line is doubled and ro
tated heteronymously to the positions m m', carrying with them the double images of the median object A as a a'. The above diagram correctly represents the position and the distance of the double images a a', and the position of the combined image B, but can not represent the distance of the combined image, because there is no point of sight. For the point of sight is really the point of optic convergence or meeting of visual lines; in diagrams representing visual results, it is the point of crossing of the doubled median lines; but this point, by both definitions, would be in this case behind the head. The diagram therefore correctly represents all the visual facts; for, there being in divergence
no point of sight, the distance of objects in the visual line is indeterminate as represented. It is impossible by the usual method to correctly represent any of the visual facts.
3. If the Law of Direction be opposed to the Law of Corresponding Points, the Latter will prevail.-These two most fundamental laws of vision are sometimes in discordance with each other. The reason of this may be thus explained: The law of direction is the fundamental law of monocular vision, as the law of corresponding points is of binocular vision. Now, for each eye, and therefore for the monocular observer, direction is determined by reference to the optic axis, but for the binocular observer by reference to the median line. On account of this difference of line of reference, while objects seen single are seen in their true positions, double images are always seen in positions different, and in some cases widely different, from the object which they represent. The difference may even amount to 45°. For example: The binocular field of view in my own case is 100° in a horizontal direction. By strong convergence I can nearly bring the double images of the root of my nose together, and thus obliterate the common field. I am sure therefore that I can make the optic axes of my two eyes cross each other at right angles. In such a case, of course, objects directly in front are doubled and their images separated 90° from each other, while objects lying to the right and left 90° from each other are brought to the front and their images superposed. Here the images are 45° from the true position of the objects which they represent. Thus Fig. 143 represents the actual relation of things in this case, and Fig. 144 the visual result, showing that the positions of the objects M and a a are com