PERCEPTION OF COLORS. 



857 



The recognition of the impression of light requires lesser intensity 

 of action than does that of a color. If the colored object is exceedingly 

 small, if it is poorly illuminated, or if it is seen for only a short time, 

 it appears colorless. The different colors show different gradations of 

 activity in this respect, red furnishing the most unfavorable conditions. 

 Simple colors, for example those of the spectrum, are produced by 

 the action of a definite number of oscillations upon the retina. Mixed 

 colors are produced when the retina is stimulated by two or more simple 

 colors, either simultaneously or in rapid alternation. The most com- 

 plex mixed color is white, which is composed of all of the simple colors 

 of the spectrum. Complementary colors comprise any two whose ad- 

 mixture produces white. For the sake of completeness contrast-colors 

 must be mentioned, as they are closely related to the complemen- 

 tary colors. They comprise any two colors that when mixed produce 

 the tone of the general illumination that prevails at the time. In the 

 blue light of the sky, the two contrast-colors must yield bluish white, 

 in bright gaslight, yellowish white. In pure white illumination the 

 contrast-colors are, naturally, the same as the complementary colors. 



Methods of Mixing Colors. (i) Two solar spectra are projected upon a screen, 

 and the colors to be mixed are superposed. (2) The observer looks obliquely 

 through a vertical glass plate at a color lying behind it. A second color is placed 

 in front of the plate, so that its image is reflected by the glass into the eye of 

 the observer. In this way transmitted light from one color and reflected 

 light from the other enter the eye at the same time. (3) By means of the 

 "color-top" small sectors of various colors are rotated rapidly on a disc. 

 By rapid rotation the impressions produced by the individual colors are united 

 to produce a mixed color. If the rotating disc which yields for example, a white 

 color from the mixture of the prismatic colors, is viewed in a rapidly rotating mir- 

 ror, the individual components of the white reappear. (4) Two different colored 

 glasses are placed before the little holes in the cardboard used in Scheiner's ex- 

 periment (p. 835, Fig. 284). The colored rays of light passing through the holes 

 are united on the retina for the production of the mixed color. 



Investigation has shown that the following colors of the spectrum are com- 

 plementary, that is two together produce white: red + greenish blue; orange 

 + cyan blue; yellow + indigo blue; greenish yellow + violet. Green has 

 the compound complementary color purple. All of the mixed colors may be 

 determined from the following table. At the head of the vertical and horizontal 

 columns are placed the simple colors; the mixed color is found at the intersection 

 of the respective horizontal and vertical columns: 



Observations upon the mixing of colors have yielded the following 

 results: (i) When two simple, but not complementary colors are mixed, 

 they produce a color-sensation that may be represented by a color situated 

 between them in the spectrum, to which a certain amount of white is 



