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 black

If we must describe it as of any color, we should

say that it is a dark grayish or brownish field, full of irregular, confused, and ever-shifting lines and cloudings. If the retina has been previously strongly impressed, spectra are seen on this dark background when the eyes are shut. When the eyes are open, the same spectra are seen on the bright ground of the sky or wall, and the difference of the background makes the difference of the color of the spectra in the two cases.

Now the same inherent activity of the retina which produces the sense of a dark field with its confused markings and cloudings, will also, under certain circumstances of peculiar sensitiveness of the retina, as after complete rest in the early morning, give rise spontaneously to more definite spectra, often of beautiful colors. I have often, in bed in the morning, watched with eyes shut these splendid spectra, consisting of a colored patch surrounded with a border of complementary color, each color closing in on the center and so vanishing, while another border commences on the outside to close in in the same way. Thus, just as impressions or images made normally on the retina by actual objects from without are projected into the field of view and seen there as the true signs of objects, even so impressions made on the retina abnormally from within, by the mind or imagination, are also sometimes projected outward, and become the delusive signs of external objects having no existence. It is thus that the diseased brain gives rise to delusive visual phenomena.

Corresponding Points, Retinal and Spatial.-Further, it is evident that every point-every rod or cone-in the retina has its invariable correspondent in the visual field, and vice versa. Moreover, since the central ray of the pencil of every radiant point in the external world passes through the nodal point of the crystalline lens,

in

it is evident that these lines must cross each other there. In other words, the lines forming correspondent points space and on the retina cross each other in the nodal point, and therefore the positions of these correspondent points, external and internal, are completely reversed. Thus not only are the retinal images inverted, but the relative positions of these images are inverted, and the position of every focal point is the inverse of its correspondent radiant point. It is obvious, then, that the left half of the retina corresponds with the right half of the field of view, and the right half of the former to the left half of the latter; and so also the upper half of the former corresponds to the lower half of the latter, and the lower half of the former to the upper half of the latter.

There are some peculiarities of vision which we are now prepared to explain.

1. Properties of the Central Spot, and of its Representative in the Visual Field.-We have already stated that there are two spots on the retina where the constituent layers do not all exist. The central spot is destitute of all except the bacillary layer; the blind spot, of all except the fibrous layer.

The central spot (macula centralis) is a small depression not more than one thirtieth of an inch in diameter, situated directly in the axis of the eye, or what might be called the south pole of this globe. It differs from other parts of the retina (a) by wanting the fibrous and granular layers; therefore the retina is much thinner there, and the spot is consequently pit-shaped, and on this account is often called the fovea centralis, or central pit. Of course, the absence of other layers exposes the bacillary layer here to the direct action of light. It differs again (6) by the presence of a pale-yellow coloring