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some trouble in convincing a few persons, and have found one single person whom I could not convince, that there were two images. To such a person all that I am about to say on binocular vision will be utterly unintelligible. The whole cause of the difficulty in perceiving at once double images is, that we habitually neglect one image unless attention is especially drawn to it. I have found that nearly all persons neglect the right-hand image-i. e., the image belonging to the left eye (unless the right eye is defective). In other words, they are right-eyed as well as righthanded. I have also tried the same experiment on several left-handed persons, and have found that these neglected the left image—i. e., the image belonging to the right eye. In other words, they were left-eyed as well as left-handed. There is no doubt that dextrality affects the whole side of the body, and is the result of greater activity of the left cerebral hemisphere. People are right-handed because they are left-brained.
I pause a moment in order to draw attention here to the uncertainty of some so-called facts of consciousness. I have often labored to convince a person, unaccustomed to analyze his visual impressions, of the existence of double images in his own case. He would appeal with confidence, perhaps with some heat, to his consciousness against my reason; and yet he would finally admit that I was right and he was wrong. So-called facts of consciousness must be scrutinized and analyzed, and subjected to the crucible of reason, as well as other supposed facts, before they should be received.
Experiment 3.-Place the two forefingers, one before the other, in the middle plane of the head (i. e., the vertical plane through the nose, and dividing the head into two symmetrical halves), and separated by a
considerable distance-say one 8 inches and the other 18 to 20 inches from the eyes. Now, if we look at the farther finger, it will be of course seen single, but the nearer one is double; if we look at the nearer finger, this will be seen single, but the farther one is now double; but it is impossible to see both of them as single objects at the same time. By alternately shutting one eye and then the other, we can observe in either case which of the double images disappears. Thus we will learn that when we look at the farther finger, the nearer one is so doubled that the left image belongs to the right eye and the right image to the left eye; while, on the contrary, when we look at the nearer finger, the farther one is so doubled that the right image belongs to the right eye and the left image to the left
eye. In the former case the images are said to be heteronymous, i. e., of different name, and in the latter case they are said to be homonymous, i. e., of the same name, as the eye. Experiment 4.-Instead of a narrow object like the finger, take next some object wider than the distance between the eye-centers-such as a postal card, for example-and repeat experiment 1. While we look at the wall the card doubles, but the double images do not entirely separate. There is a middle opaque overlapping part with shadowy transparent margins right and left (Fig. 36). In this figure a b c d is the right-eye image and a' b' c' d' the left-eye image. The overlapping part is opaque, because it covers a part of the wall
hidden from both eyes. The margins are transparent, because they cover portions of the wall hidden from one eye but seen by the other. As we gaze steadily we observe a sort of struggle between the two images for mastery. First perhaps the right-eye image prevails, the left-eye image disappearing and the right-eye image becoming opaque throughout. Then the left-eye image prevails and the reverse takes place.
There is a limit, therefore, to the separation of double images when we look beyond the object-i. e., in case of heteronymously double images. This limit is the interocular space, and the reason is that we can not turn our eyes outward beyond parallelism. There is no limit in the case of homonymously double images except the ability to converge the optic axes.
It is evident, then, that double images are formed whenever the optic axes are not turned directly on the object observed. For example: if the finger be pressed in the corner of one
or both eyes we see double images. If it is the external corner, the images are heteronymous; if the internal corner, they are homonymous.
Analogues of Double Images in Other Senses. Whenever it was possible, we have traced the analogy of visual phenomena in other senses. Is there any analogue of double vision to be found in other senses? There is, as may be shown by the following experiment: If we cross the middle finger over the forefinger until the points are well separated, and then roll a small round body, like a child's marble, about on the table between the points of the crossed fingers, we will dis
tinctly perceive two marbles. The points of the fingers touched by the marble are unaccustomed to be touched in that way-they are non-corresponding. (Fig. 37.)
Single Vision. Therefore it is evident that when we look directly at anything we see it single, but that all things nearer or beyond the point of sight are seen double. We then come back to our previous proposition, that we always see things double except under certain conditions. What, then, are the conditions of single vision? I answer: We see a thing single when the two images of that thing are projected outward to the same spot in space, and are therefore superposed and coincide. Under all other conditions we see them double. Again: the two external images of an object are thrown to the same spot, and thus superposed and seen single, when the two retinal images of that object fall on what are called corresponding points (or sometimes identical points) of the two retina. If they do not fall on corresponding points of the two retinæ, then the external images are thrown to different places in space, and therefore seen double. We must now explain the position of corresponding points of the two retinæ.
Corresponding Points. The retinæ, as already seen, are two deeply concave or cup-shaped expansions of the optic nerve. If R and L, Fig. 38, represent a. projection of these two retinal cups, then the black spots CC", in the centers of the bottom, will represent the position of the central spots. If now we draw vertical lines (vertical meridians), a b, a' b', through the central spots, so as to divide the retina into two equal halves, then the right or shaded halves would correspond point for point, and the left or unshaded halves would correspond point for point; i. e., the internal or nasal half of one
retina corresponds with the external or temporal half of the other, and vice versa. Or, more accurately, if the concave retina be covered with a system of rectangular spherical coördinates, like the lines of latitude and longitude of a globe, a b and ay being the meridian and equator, then points of similar longitude and latitude in the two retinæ, as d d', e e', are corresponding. Or, still better, suppose the two eyes or the two retina to be placed one upon the other, so that they coincide throughout like geometric solids; then the coincident points are also corresponding points. Or again: Take
a pair of dividers and open the points until they are the exact distance apart of the central spots (interocular space). Then, holding level, suppose the two retinæ to be touched at many points. The points touched at the same time would be corresponding points. The mode of getting the interocular space is fully described on page 265. It is usually about two and a half inches. Of course, the central spots will be corresponding points; also points on the vertical meridians, a b, a' b', at equal distances from the central spots, will be corresponding ; also points similarly situated in similar quadrants, as dd', e e', etc. It is probable that the definition just given