perceiving at once double images is, that we habitually neglect one image unless attention is specially drawn to it. I have found that nearly all persons neglect the right-hand image-i. e., the image belonging to the left eye. In other words, they are right-eyed as well as right-handed. 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. Socalled 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.

Is

FIG. 80.

Analogues of Double Images in Other Senses.-Whenever it was possible, we have traced the analogy of visual phenomena in other senses. 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 distinctly perceive two marbles. The points of the fingers touched by the marble are noncorresponding. (Fig. 30.)

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 proposi

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tion, 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 retina, 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 retina, as already seen, are two deeply concave or cup-shaped expansions of the optic nerve. If R and L, Fig. 31, represent a projection of these two retinal cups, then the black spots C C'.

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 halves would correspond point for point, and

the left 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 retinæ be covered with a system of rectangular spherical coördinates, like the lines of latitude and longitude of a globe, ab and x y 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 retinæ to be placed one upon the other, so that they coincide throughout like geometric solids; then the coincident points are also corresponding points. 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 d d', e e', etc. It is probable that the definition just given is not mathematically exact for some eyes. It is probable that in some eyes the apparent vertical meridian which divides the retinæ into corresponding halves is not perfectly vertical, but slightly inclined outward at the top. This would affect all the meridians slightly; but the effect is very small, and I do not find it so in my eyes. We shall discuss this point again (page 146).

Law of Corresponding Points.-After this explanation we reënunciate the law of corresponding points: Objects are seen single when their retinal images fall on corresponding points. If they do not fall on corresponding points, their external images are thrown to different places in space, and therefore are seen double.

Thus we see that the term "corresponding points" is used in two senses, which must be kept distinct in the mind of the reader. Every rod and cone in each

retina has its correspondent in external space, and these exchange with each other by impression and projection. Also every rod or cone of each retina has its correspondent in a rod or cone in the other retina. Now the law of corresponding points, with which we are now dealing, states that the two external or spatial correspondents of two retinal corresponding points always coin

R and L, two eyes; 0, center of rotation of ball, or optic center; ∞, point of crossing of ray-lines-nodal point; A, point of sight; D, some other point in the horoptoric circle A 0 0'; c c', central spots; a a', d d', actual images of A and D.

cide with each other. In order to distinguish these two kinds of corresponding points from each other, the latter-i. e., corresponding points on the two retina-are often, and perhaps best, called "identical points," because their external spatial representatives are really identical.

We will now apply the law. If we look directly at