780 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



fours, etc. Any two spectral colors which by their union produce white are 

 said to be " complementary " colors. The relation of these pairs of comple- 

 mentary colors to each other may be best understood by reference to Figure 238. 



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FIG. 238. Color diagram. 



Here the spectral colors are supposed to be disposed around a curved line, 

 as indicated by their initial letters, and the two ends of the curve are united 

 by a straight line, thus enclosing a surface having somewhat the form of a tri- 

 angle with a rounded apex. If the curved edge of this surface be supposed to 

 be loaded with weights proportionate to the luminosity of the different colors, 

 the centre of gravity of the surface will be near the point W. Now, if a 

 straight line be drawn from any point on the curved line through the point 

 W and prolonged till it cuts the curve again, the colors corresponding to the 

 two ends of this straight line will be complementary colors. Thus in Figure 

 238 it will be seen that the complementary color of red is bluish-green, and 

 that of yellow lies near the indigo. It is also evident that the complementary 

 color of green is purple, which is not a spectral color at all, but a color 

 obtained by the union of violet and red. The union of a pair of colors 

 lying nearer together than complementary colors produces an intermediate color 

 mixed with an amount of white which is proportionate to the nearness of the 

 colors to the complementary. Thus the union of red and yellow produces 

 orange, but a less saturated orange than the spectral color. The union of two 

 colors lying farther apart than complementary colors produces a color which 

 borders more or less upon purple. 



The mixing of colors to demonstrate the above-mentioned effects may be 

 accomplished in three different ways : 



1. By employing two prisms to produce two independent spectra, and then 

 directing the colored rays which are to be united so that they will illuminate 

 the same white surface. 



2. By looking obliquely through a glass plate at a colored object placed 

 behind it, while at the same time light from another colored object, placed in 

 front of the glass, is reflected into the eye of the observer, as shown in Figure 

 239. Here the transmitted light from the colored object A and the reflected 

 light from the colored object B enter the eye at C from the same direction, 

 and are therefore united upon the retina. 



3. By rotating before the eye a disk on which the colors to be united are 



