PHYSIOLOGY OF COLOR VISION 453 



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 under- 

 stand 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. 

 Eapidly 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, 2 by choosing various proportions 

 of the spectral colors this white becomes tinted with all possible inter- 

 mediate hues. 



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



2 It would be pure white were it possible to obtain artificial pigments that 

 reflected none other than their own characteristic hues. 



