Page:Popular Science Monthly Volume 83.djvu/457

Rh physiological properties for each spectral color, hue and brightness. There is, however, another property of colors as seen in nature which is absent in the spectrum, namely saturation. This refers to the degree of white light with which the color is mixed. It is more or less related to the artist's "value" which expresses the translation of the colors into gray.

The most characteristic of these properties of colors is their hue, and for the present we shall confine our attention to this. To understand what the hue is due to we must remember that rays of light exist in space as vibrations of the surrounding ether and that these vibrations occur at right angles to the line of propagation of the light rays. The rate of the vibration varies according to the hue. In other words, the light rays are made up of waves which are small and close together when the vibration is rapid, as at the violet end of the spectrum, and are large and wide apart when the vibration is slow, as at the red end. When these waves strike the retina they create impressions which differ from one another according to the wave-lengths. These differences we interpret as differences in hue. When the rays of the various spectral colors are reunited before striking the retina, the sensation which is created is that of white. This recombination of the spectral colors, which is called synthesis of colors, may in general be brought about in two ways: (1) by causing them to fuse together by means of some suitable optical device (such as a second prism, or reflecting mirrors) before they enter the eye, (2) by causing them to become superimposed upon one another on the retina in rapid succession, in which case the impression created by each color lasts for a sufficient length of time so that it becomes fused with those which succeed it. This result depends on the phenomenon of positive after-images; which can be demonstrated by momentarily regarding some brightly illuminated object and then closing the eyes, when the image continues to be seen for some time. Rapidly succeeding images therefore become fused into one composite impression. This retinal synthesis, as we may call it, is well illustrated in the impression produced by observing the spokes of a rapidly revolving wheel.

For experimental purposes it is brought about by using Maxwell's machine, which consists of circular cards painted in sectors with the various colors and which are caused to revolve around their centers by means of a motor. A spinning top may also be used for this purpose. By revolving a card painted with the seven spectral colors a sensation approaching that of white is produced, by choosing various proportions of the spectral colors this white becomes tinted with all possible intermediate hues.

From these facts we might imagine that the retina contains a special