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 spec-tram. The white solar light is thus shown to be a compound of rays of several colors which possess different degrees of refrangi-bility, 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 or diminished activity. A varying degree of these stimulations can be easily shown to give many differences of shade.

Diagram of the three Primary Sensations. 1 = red; 2 = green; 3 = violet. The letters below are the initials of the colors of the spectrum.

Fig. 235. Diagram of the three Primary Sensations. 1 = red; 2 = green; 3 = violet. 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 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 5° 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.