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the two eyes with their two visual lines seem to unite to form a single middle binocular eye, and a single middle visual line, along which the eye seems to look. It follows that any line, rod, or plane in the median line, as also the nose itself, is doubled heteronymously, and becomes two lines, rods, or planes, parallel to each other, and separated by a space exactly equal to the interocular space. Between the two noses, and between the two parallel lines, rods, or planes, the binocular eye seems to look out along the middle visual line upon the distant object. Of course, by this shifting of the twofields in opposite directions, all objects in the field are similarly doubled.
Thus in binocular vision the two eyes seem actually to be brought together and superposel, and corresponding points of the two retinæ to coincide. The two eyes become actually one instrument. And conversely, this apparent combination of two eyes and their visual lines is a necessary consequence of the law of corresponding points. For images on corresponding points are seen single; all objects on the two visual lines must impress corresponding points, viz., the central spots ; therefore the visual lines themselves, if they were visible lines, would be seen single. But where could they be seen single except in the middle? Therefore the two visual lines must combine to form a single middle visual line.
We will next give experiments leading up to the second law. For this purpose let us recur to the experiment with the rod represented by Fig. 98. We reproduce this as Fig. 101, only simplifying by leaving out, in the visual result (Fig. 102), the monocular visual line vu' of Fig. 99, in order to compare with it the results of subsequent experiments. As already explained, if the rod B be placed in the median plane with the nearer en resting on the nose-root n, and the farther
end held in place by the point of the finger A, the eyes looking at a distant object, as shown in Fig. 101, which represents the actual condition of things, then the rod,
together with nose and finger-point, will be doubled heteronymously and become two parallel rods, between which the binocular eye will look out along the binocular visual line at the distant object, as shown in Fig. 102, which represents the visual results.
Experiment 1.- Now, while we hold the rod in the position represented by Fig. 101, instead of looking at a distant object with eyes parallel, let the eyes be converged on the finger-point F, so that Fig. 103 shall represe.it the actual condition of things. We will observe
that the double images of the rod represented in the visual result, Fig. 102, approach at their farther end, carrying all objects in the field with them, until they unite at the point of sight F, and we have the visual result represented in Fig. 104.
Ecperiment 2.- If by greater convergence we next look at some nearer point B on the rod, as in Fig. 105, which represents the actual relation of parts, then Fig. 106 represents the visual result. By comparing this withi the previous visual results, Figs. 102 and 104, it will be seen that the double images b b' approach each other until they unite at the point of sight, and the two inages of the rol cross each other at this point, and therefore become again double beyond, but now homonymously. If by still greater convergence we look at a still nearer point C, Fig. 107, then the double images of the median rod, Figs 101, 103, 105, will cross at the point of sight C, and give the visual result shown in Fig.
108. Finally, if the point of sight by extreme convergence be brought to the root of the nose, then the double images of the nose n n', Figs. 106, 108, will be brought in contact, and the common or binocular field will be obliterated. In all cases it will be observed that the combined eyes look along the combined visual lines through the point of sight, and onward to infinite distance.
It is evident, then, that in optic convergence, as the two real eyes turn in opposite directions on their optic centers, the two fields of view turn also on the center of the binocular eye in directions opposite to the real eyes, and therefore to each other.
It will be observed that in speaking of visual phenomena I have used much the same language as other writers on this subject, and used also a somewhat similar mode of representation; only I have substituted eyes in the place of the nose, and put noses in the position of the eyes.
I have made median lines cross each other at the point of sight, instead of visual lines, and visual lines combine in the middle as a true median visual line. In other words, I have used the true language of binocular vision. I have expresse:1 what we see, rather than what we know—the language of simple appearance, rather than that inixture of appearance and reality which forms the usual language of writers on this subject.
Second Law.-The second law may therefore le stated thus : In turning the eyes together in the same direction, without altering their convergence, objects seem stationary, and the visual lines seem to move and sweep over them; but when we turn the eyes in opposite directions, as in increasing or decreasing their convergence, then the visual lines seem stationary (i. e., we seem to look in the same direction straight forward), and all objects, or rather their images, seem to move in directions contrary to the actual motion of the eyes. The whole fields of view of both eyes seem to rotate about a middle optic center, in a direction contrary to the motion of the corresponding eyes, and therefore to each other. This is plainly seen by voluntarily and strongly converging the eyes on an imaginary very near point, as for example the root of the nose, and at the same time watching the motion of the images of more