to change their shape slightly, and definitely to alter the position of the pigment granules they contain when exposed to light. When we remember how sensitive to light the protoplasm of many unicellular infusoria is, we cannot be surprised that the protoplasm of the retinal epithelium is affected by it. In the pigment cells of the frog's skin we are familiar with a change in shape and in the arrangement of their pigment granules in response to different light stimuli. We know further that in the nervous centres nerve impulses often originate in protoplasm under the influence of slight changes in temperature. or nutrition. It would hardly be too much to assume, then, that the retinal epithelium has some important share in the transformation of light into a nerve stimulus. The arguments pointing to the

Epithelial cells of the retina. a, Seen from the outer surface; b, seen from the side as in a section of the retina; c, shows some rods projecting into the pigmented protoplasm.

rods and cones as the essential part of the retina apply equally well to the pigmented epithelium, for they are so dove-tailed one into the other that practically they form but one layer. They are not known to be connected with the nerve fibres, but they may still be influenced by the light, and communicate the effect to the contiguous nerve terminals, which appear to be elaborately adapted to the appreciation of subtle forms of stimulation.

COLOR PERCEPTIONS.

If a beam of white sunlight be allowed to pass through an angular piece of glass it is decomposed into a number of colors which may be seen by looking through the prism, or may be thrown on a screen, like that of a camera. These colors, which

look like a thin slice of a rainbow, are together called the spectrum. The white solar light is thus shown to be a compound of rays of several colors which possess different degrees of refrangibility, and hence are separated on their way through the prism. The violet rays are the most bent, and the red the least, so that these form the two extremes of the visible spectrum. The difference of color depends upon the different lengths of the waves, the vibrations of violet (762 billions per sec.) being much more rapid than those of red (394 billions per sec.). Beyond the visible spectrum at the red end there are other rays which, though they look black to the eye, are capable of transmitting heat. This thermic power is best developed in these ultra-red rays and fades gradually toward the middle of the spectrum. Outside the violet are ultra-violet rays, which, though non-exciting to the retina, are very active in inducing many chemical changes. Only those ether vibrations which have a medium length can stimulate the retina.

If two different colors be mixed before reaching the retina, or be applied to it in very rapid succession one after the other, an impression is produced which differs from both the colors when looked at separately; thus, violet and red give the impression of purple, a color not in the spectrum. If all the colors of the spectrum in the same proportion and with the same brightness fall upon the retina, the result is white light. This we know from the common experience of ordinary white light, which is really a mixture of all the colors of the spectrum, and we can see it with a "color top" painted to imitate the colors of the spectrum. When the top is spinning, the colors meet the eye in such rapid succession that the stimulus of each falls on the retina before that of the others has faded away, and thus many colors are practically applied to the retina at the same time, and the top looks nearly white.

It has been found that certain pairs of colors taken from the spectrum when mixed in a certain proportion produce white. These are complementary to one another. The complementary colors are:

Red and peacock-blue.
Orange and deep blue.

Yellow and indigo.
Greenish-yellow and violet.

If colors which lie nearer to each other in the spectrum than these complementary colors be mixed, the result is some color which is to be found in the spectrum between the two mixed.

The perception of the vast variety of shades of color that we can distinguish can only be explained by means of this color mixing. We may suppose (with Hering) that there are three varieties of material in the retina, each of which gives rise to antagonistic or complementary color sensations according as they undergo increased or decreased molecular activity, these antagonistic states being produced by the complementary colors. Thus, one substance gives the sensation of black or white, another red or green, another yellow or blue, according as they are in exalted

red; 2

green;

3= violet.

Diagram of the three Primary Sensations: 1 The letters below are the initials of the colors of the spectrum. The height of the shaded part gives extent to which the several primary sensations are excited by different kinds of light in the spectrum.

or diminished activity. A varying degree of these stimulations can be easily shown to give many differences of shade.

Or we may assume that there are three primary colors which overlap one another in the spectrum so as to produce all the various tints. These are red, green, and violet; the arrangement of which may be diagrammatically explained (Fig. 235).

We must in this case further assume (Young, Helmholtz) that there are in the retina three special sets of nerve terminals, each of which can only be stimulated by red, green, or violet respectively, and the innumerable shades of color we see depend upon

mixtures of different strengths of these primary colors, producing different degrees of stimulation of each set of nerve terminals.

The view that such special nerve apparatus exists for red, green and violet is supported by the fact that the most anterior or marginal part of the retina is incapable of being stimulated by red objects, which look black when only seen by this part of the retina. This inability to see red may extend over the whole retina, as is found in some persons who may be said to be "red blind." If we investigate our negative after images, after looking for a long time at a red object, we find them to be greenish blue. That is to say, the nervous mechanism for receiving red impressions is fatigued, and that of its complementary color is easily stimulated.

MENTAL OPERATIONS IN VISION.

Our visual sensations enable us to perceive the existence, position and correct form of the various objects around us. For visual perception much more is necessary than the mere perfection of the dioptric media of the eye, and of the retinal nerve mechanisms. Besides the changes produced in the retina by light and the excitations in the nerve cells of the visual centre, there must be psychical action in other cells of the cortex of the brain. This psychical action of the brain consists of a series of conclusions drawn from the experiences gained by our visual and other sensations.

Our ideas of external objects are not in exact accord with the image produced on the retina and transmitted to the brain, but are the result of a kind of argument carried on unconsciously in our minds. Thus, when no light reaches the retina, we say (without what we call thought) that it is dark; our retina being unstimulated, no impulse is communicated, and the sensation of blackness arises in our sensorium. When luminous rays are reflected to the retina from various objects around us, the physiological impulse starts from the eye, but in the brain, by unconscious psychical activity, it is referred in our minds to the objects around us, so that mentally we project into the outer world what really occurs in the eye. So also, from habit, we re-invert in our

minds the image which is thrown on the retina upside down, by the lens, and so unconscious are we of the psychical act that we find it hard to believe that our eyes really receive the image of everything inverted, and our minds have to reinstate it to the upright position.

One of the most important means employed to enable us to form accurate visual perceptions is the varied motion which the eyeballs are capable of performing.

MOVEMENTS OF THE EYEBALLS.

obl.sup.

r.inf

FIG. 236.

obl.sup.

ahl inf

r.sup

The eyeballs may be regarded as spherical bodies, lying in loosely fitted sockets of connective tissue padded with fat, in which they can move or revolve freely in all directions, in a limited degree. The muscles which act directly on the eyeball are six in number. Four recti passing from the back of the orbit are attached to the eyeball, one at each side and one above and below, not far from the cornea. These move the front of the eye to the right or left, up or down respectively. Two oblique passing nearly horizontally outward, and a little backward, are attached to the upper and under surface of the eyeball respectively. These muscles can slightly rotate the eye on its antero-posterior axis, the upper one drawing the upper part of the eyeball inward, and its antagonist, the lower, drawing the lower part inward, so as to rotate the eyeball in an opposite direction round the same axis.

obl. inf.

r.ext. r.sup. r.int r.inf.

Diagram of the direction of action of the muscles of the eyeball, which is shown by the dark lines. The axes of the rotation caused by the oblique and upper and lower recti are shown by the dotted lines. The inner and outer recti rotate the ball on its vertical axis, which is cut across. The abbreviated names of the muscles are affixed to the lines.

The internal and external recti draw the centre of the cornea