the two retina. These two fields of view partly overlap each other, so as to form a common or binocular field. Fig. 29 represents roughly the form of these fields in my own case. The right field, R, is bounded by the line of the nose n n on the left, the brows br above, and the cheek ch below. The field of the left eye, L, is bounded similarly on the right by the nose n'n', n' n', the brow br', and the cheek ch'. Between the lines of the nose, n n, n'n', is the rounded triangular space CF, which is the common or binocular field. This common field is the only part seen by both eyes. The two fields are left vacant on the extreme right and left, because, projected on a plane surface, they are unlimited in these directions. This is the necessary result of the fact that in a horizontal direction the field of view of both eyes is more than 180°.

Now, there being two retina, there are of course two retinal images of every external object; and since retinal images are projected outward into space as external images, we must have two external images of every object. But we see objects only by these external images. Why, then, with two retinal images-ay, and two external images for every object, do we not see all objects double? I answer: We do indeed see all objects double, except under certain conditions.

Double Images.—This phenomenon of double images of all objects, except under certain special conditions, is so fundamental in binocular vision, and yet so commonly overlooked by even the most intelligent persons unaccustomed to analyze their visual impressions, that it becomes absolutely necessary first of all to prove it by detailing many experiments, which every one may repeat for himself.

Experiment 1.-Holding up the finger before the

eyes, look, not at the finger, but at the wall or the ceil ing or the sky. Two transparent images of the finger will be seen, the left one belonging to the right eye and the right one to the left eye. We easily prove this by shutting first one and then the other eye, and observing which image disappears. The images are transparent, or shadowy, because they do not conceal anything. The place covered by the right-eye image is seen by the left eye, and the place covered by the lefteye image is seen by the right eye. If we alternately shut one eye and then the other, the wide difference between these places is at once evident. Often there is an alternation in the distinctness of these shadowy images-first one and then the other fading away, and almost disappearing from view.

Experiment 2.-Point with the forefinger at some distant object, looking with both eyes open at the object, not the finger. Two fingers will be seen, one of them pointing at the object and the other far out of range, usually to the right."

Most persons find some difficulty at first in being conscious of perceiving two images. The reason is, they do not easily separate what they know from what they see. They know there is but one finger, and therefore they think they see but one. The best plan is to shut alternately one eye and then the other, and observe the places of projection of the finger against the wall; and then, opening both eyes, shadowy images at both these places will be seen. I have found 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 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

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 retinæ, 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 retinæ into two equal halves, then the right halves would correspond point for point, and