this experiment can not be easily made. It has been made, however, by passing a spark of electricity through the head or through the eye in such wise as to penetrate the retina or traverse the optic nerve. The phenomenon has also been observed in cases of extirpation of the eye at the moment of section of the optic nerve. (Helmholtz.)

Experiment 2. Phosphenes.-Press the finger into the internal corner of the eye: you perceive a brilliant colored spectrum in the field of view on the opposite or external side. The spectrum thus produced has a deep steel-blue center, with a brilliant yellow border, and reminds one of the beauty spots on a peacock's feather or a butterfly's wing. Remove the pressure to any other part, and the spectrum moves also, but retains its opposite position in the field of view. In this familiar experiment the pressure indents the sclerotic and causes a change or irritation on the forward portion of the retina; and any change whatever on the retina is always referred directly outward at a right angle to the point impressed, and therefore to the opposite side of the field of view. These colored spectra have been called phosphenes.

Experiment 3. Musca Volitantes.—If we gaze on a white wall or ceiling, or, still better, on a bright sky, we see indistinct motes floating about in the field of view on the wall or sky, and slowly gravitating downward. Sometimes they are undulating, transparent tubes, with nucleated cells within; sometimes they are like inextricably tangled threads, or like matted masses of spider's web; sometimes they are slightly darker spots, like faint clouds. They are called musca volitantes, or flying gnats. What are they? They are specks or imperfections in the transparency of the vitreous

humor. As fishes or other objects floating in midwater of a clear lake on a sunny day cast their shadows on the bottom ooze, even so these motes in the clear medium of the vitreous humor, in the strong light of the sky, cast their shadows on the retinal bottom. Now, as already said, all changes in the retina, of whatever kind, whether produced by images, or shadows, or mechanical irritations, are projected outward into the field of view, and appear there as something visible.

Experiment 4. Purkinje's Figures.-Stand in a dark room with a lighted candle in hand. Shutting the left, hold the candle very near the right eye, within three or four inches, obliquely outward and forward, so that the light shall strongly illuminate the retina. Now move the light about gently, upward, downward, back

FIG. 21.

and forth, while you gaze intently on the wall opposite. Presently the field of view becomes dark from the intense impression of the light, and then, as you move the light about, there appears projected on the wall and covering its whole surface a shadowy, ghostlike image, like a branching, INTERNAL VIEW OF THE RETINA, leafless tree, or like a great

ing the central spot. (After
Cleland.)

fying over the surface, but avoid- bodiless spider with many branching legs. What is it? It is an exact but enlarged image of the blood-vessels of the retina (Fig. 24). These come in at the entrance of the optic nerve, ramify in the middle layer, and therefore in the strong light cast their shadows on the bacillary layer, of the retina. The impression of these shadows is projected outward into

the field of view, and seen there as an enlarged shadowy image. These have been called Purkinje's figures, from the discoverer.

Experiment 5. Ocular Spectra.—Look a moment steadily at the setting sun, and then, turning away the eye, look elsewhere-at the sky, the ground, the wall: a vivid colored spectrum of the sun (or many of them, if the eye has not been steady while regarding the sun) is projected into the field of view, and follows all the motions of the eye. This spectrum, on a bright ground, like the sky, to my eye is first green, then blue, then purple, and so gradually fades away. The spectrum is equally seen when the eye is shut; but then, being projected on a dark ground, the color is apt to be complementary to that of the same spectrum seen against the bright ground of the sky. It is first blue, then yellow, then green, and so fades. The explanation is obvious. The strong impression of the image of the sun on the retina induces a change which lasts some time; but every change in the retina appears, by projection, in the field of view.

This experiment may be made in an infinite variety of ways. If at night we gaze steadily at a candle- or lamp-flame, or flame of any kind, and then turn away and look at the wall, we see a vivid colored spectrum of the flame, which gradually changes its color and fades away. In my own case, on shutting the eyes, the spectrum is first bright yellow, with deep-red border and dark olive-green corona; then it becomes greenishyellow, and then green with red border, then red with indigo border, and so fades away. With the eyes open. the changes are slightly different, and in some stages are complementary to the preceding. Again, if we look a moment through a window at a bright sky, and

then quickly turn the eye to the wall, we will see a faint spectrum of the window with all its bars projected against the wall. If we look intently and steadily at any object strongly differentiated from the rest of the wall of a room, as a small picture-frame or a clock, then look to some other part of the wall, the spectrum of the object will be seen on the wall and follow the eye in its motions. This experiment succeeds best when we are just waked up in the morning, and while the retina is still sensitive from long rest.

The experiment may be varied thus: Lay a small patch of vermilion red-such as a red wafer-on a white sheet of paper, and gaze steadily at it in a strong light for a considerable time, and then turn the eye to some other part of the paper. A spectrum of the wafer will be seen, because every difference in the retina will appear as a corresponding difference in the field. It will be observed, also, that the spectrum will be bluish-green, i. e., complementary to the red of the object. The reason seems to be that the long impression of the red produces a profounder change, or fatigue, in those rods or cones, or those portions of the cones, which co-vibrate with red; therefore, when we look elsewhere, of the different colors which make up white light, the retina is least sensitive to red, and therefore the other rays will predominate. Now these other rays, which with red make up white light, are what are called complementary to red. A mixture of these makes a bluish-green. It is difficult, however, to account for all the phenomena of the colors of spectra by this "law of fatigue."

Complementary spectra may be still more beautifully seen by gazing on the brilliant contrasted colors of a stained-glass window, and then turning the eyes

on a white wall. The whole pattern of the window will be distinctly seen in complementary colors.

Let it be observed here how differently spectral images behave from objects. When we move the eyes about, the images of objects move about on the retina, but the objects seem to remain unmoved. Spectral impressions on the retina, on the contrary, remain in the same place, and therefore their external images follow the motions of the eye.

We are now prepared to generalize from these observations. It is evident that what we call the field of view is naught else than the external projection into space of retinal states. All variations of state of the one, whether they be images, or shadows, or mechanical irritation, whether they be normal or abnormal, are faithfully reproduced as corresponding variations of appearances in the other. This sense of an external visual field is ineradicable. If we shut our eyes, still the field is there, and still it represents the state of the retina. With the eyes open, we call it the field of view, filled with objects; with the eyes shut, it is the field of darkness—visible, palpable darkness, without visible objects. The one is the outward projection of the active state of the retina, crowded with its retinal images; the other is the outward projection of the comparatively passive state of the retina, without definite images. When we shut our eyes, or stand with eyes open in a perfectly dark room, the field of darkness is an actual visible field, the outlines of which we can, at least imperfectly, mark out. It is wholly different from a simple absence of visual impression. We see a dark field in front, but nothing at all behind the head. The dark field is also quite different from blackIf we must describe it as of any color, we should

ness.