726 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1913. 



blue (indigo), and violet, the various shades of purple being entirely 

 absent. When we look at such a spectrum we are at once struck with 

 the fact that the colors differ from one another not only in their 

 hue, but in their brightness or luminosity, the yellow and the immedi- 

 ately adjacent portions being much brighter than the others. At 

 once, then, we recognize two physiological properties for each spec- 

 tral color — hue and brightness. There is, however, another prop- 

 erty of colors as seen in nature Avhich is absent in the spectrum, 

 namely, saturation. This refers to the degree of white light Avith 

 which the color is mixed. It is more or less related to the ai-tist'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 atttention 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 tlie various spectral hues are reunited before strik- 

 ing the retina the sensation which is created is that of white. This 

 recombination or synthesis of the spectral hues 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 posi- 

 tive after images, which can be demonstrated by momentarily re- 

 garding some brightly illuminated object and then closing the eyes, 

 when the image continues to be seen for some time. Kapidly suc- 

 ceeding images, therefore, become fused into one composite impres- 

 sion. 



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 ta 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 



