more, then light from the pin would be brought to focal points and produce an image on the retina; and this image, being inverted, would by projection be reinverted, and the pin would be seen in its real position. In the above experiment, however, the pin is much too near the retina to form an image. But nearness to the retinal screen, though unfavorable for producing an image, is most favorable for casting a sharp shadow; and while retinal images are inverted, retinal shadows are erect. The light streaming through the pin-hole into the eye casts an erect shadow of the pin-head on the retina. This shadow is projected outward into space, and by the law of direction is inverted in the act of projection, and therefore seen in this position in the pin-hole. It is further proved to be the outward projection of a retinal shadow by the fact that, by multiplying the pin-holes or sources of light, we multiply the shadows, precisely as shadows of an object in a room are multiplied by multiplying the lights in the room.*

Experiment 2.-If we look at a strong light, such as the flame of a candle or lamp, or a gas-flame, at some distance and at night, and thus bring the lids somewhat near together, we observe long rays streaming from

FIG. 27.

the light in many directions, but chiefly upward and downward. Fig. 27 gives the phenomenon as I see it. The explanation is as follows: In bringing the lids near

"

* This phenomenon was first explained by the author in 1871. See Philosophical Magazine,” vol. lxi, p. 266.

together, the moisture which suffuses the eye forms a concave lens, as in Fig. 28 (hence the phenomenon is much more conspicuous if there be considerable moisture in the eyes). This watery lens will be saddle-shapedi. e., concave vertically and convex horizontally. Now the rays from the light (L, Fig. 27) which penetrate the center of the pupil will pass directly on without refraction except what is normal, and make its image (Fig.

28, L') on the central spot. But the rays which strike the curved surface of the watery lens will be bent upward to b and downward to a. Thus the light, instead of being brought to a focal point, is brought to a long focal line, b a, on the retina, with the image of the light in the middle at L'. The upper portion of this line b L' will be projected outward and downward, and form the downward streamers of Fig. 27; while the lower portion of the retinal impression a L' will be projected outward and upward, and form the upward streamers of Fig. 27. To prove this, while the streamers are conspicuous, with the finger lift up the upper lid: immediately the lower streamers disappear; now press down the lower lid: immediately the upper streamers

disappear. Also, by shutting alternately one eye and the other, it will be seen that a b (Fig. 27) belongs to the right eye and a' b' to the left.

The much lighter diverging side-rays are more difficult to account for. I attribute them to the slight crinkling of the mucus covering the cornea in bringing the lids together.

PART II.

BINOCULAR VISION.

CHAPTER I.

SINGLE AND DOUBLE IMAGES.

The Two Eyes a Single Instrument.-We have thus far treated only of the phenomena of monocular vision; and all that we have said might still apply, almost word for word, if, like the Cyclops Polyphemus, we had but one eye in the middle of the forehead. But we have two eyes; and these are not to be considered as mere duplicates, so that if we lose one we still have another. On the contrary, the two eyes act together as one instrument; and there are many visual phenomena, and many judgments based upon these phenomena, which result entirely from the use of two eyes as one instruThese form the subject matter of Binocular Vision. It must be clearly understood that the distinctive phenomena of binocular vision require two eyes acting as one. We might have two eyes, or even, like Argus, a hundred eyes, and yet not enjoy the advantages of binocular vision; for each eye might see independently. This would still be monocular vision.

ment.

The phenomena of binocular vision are far less purely physical than those of monocular vision. They

are also far more obscure, illusory, and difficult of analysis, because far more subjective and far more closely allied to psychical phenomena. From early childhood I have amused myself with experiments in this field, and have thus acquired an unusual voluntary power over the movements of the eyes, and a still more unusual power of analysis of visual phenomena. This has always therefore been a favorite field for me; but with a little practice any one may acquire similar power and enjoy a similar pleasure.

Binocular Field.-We have said that the field of view is naught else than an outward projection of retinal states. With the eyes open and the retina in an active or stimulated condition, we call it the field of view; with the eyes shut and the retina in a comparatively passive or unstimulated condition, we call it the field of darkness. In either case, every variation in the state of different parts of the retina, whether by

shadows or by images, or by its own internal changes or unstimulated activity, is faithfully represented in external space by spectra, external images, etc. But we have two eyes, and therefore two retina, and therefore also two fields of view, the external projections of