PROPERTIES OF THE RETINA. 323 



fers to the color of the air ; green from the same root as grow, referring to the 

 color of vegetation. Yellow seems to be derived from the same root as gold, 

 which typified the color. The other less distinct qualities have names of 

 recent application, such as orange, violet, indigo blue, etc. 



Complementary Colors. It has been found by the methods of 

 color fusion that certain pairs of colors when combined give a white 

 (gray) sensation. It may be said, in fact, that for any given color 

 there exists a complement such that the fusion of the two in suitable 

 proportions gives white. If we confine ourselves to the spectral 

 colors we recognize such complementary pairs as the following: 



Red and greenish blue. 



Orange and cyan blue. 



Yellow and indigo blue. 



Greenish yellow and violet. 

 The complementary color for green is the extraspectral purple. 

 Colors that are closer together in the spectral series than the 

 complementaries give on fusion some intermediate color which is 

 more saturated that is, less mixed with white sensation the nearer 

 the colors are together. ' Thus, red and yellow, when fused, give 

 orange. Colors farther apart than the distance of the comple- 

 mentaries give some shade of purple. On the physical side, there- 

 fore, we can produce a sensation of white in two ways : Either by the 

 combined action of all the visible rays of the spectrum (sunlight) 

 or by the combined action of pairs of colors whose wave lengths vary 

 by a certain interval. It is probable that in the retina the processes 

 induced by these two methods are qualitatively the same, the 

 wave-lengths represented by the complementary colors setting up 

 by their combined action the same photochemical processes that 

 normally are induced by the sunlight. 



After-images. As the name implies, this term refers to images 

 that remain in consciousness after the objective stimulus has ceased 

 to act upon the retina. They are due doubtless to the fact that the 

 changes set up in the retina by the visual stimulus continue, with 

 or without modification, after the stimulus is withdrawn. After- 

 images are of two kinds: positive and negative. In the positive 

 after-images the visual sensation retains its normal colors. If one 

 looks at an incandescent electric light for a few seconds and then 

 closes his eyes he continues to see the luminous object for a con- 

 siderable time in its normal colors. Objects of much less intensity 

 of illumination give positive after-images, especially when the 

 eyes have been kept closed for some time, as, for instance, upon 

 waking in the morning. In negative after-images the colors are all 

 reversed, that is, they take on the complementary qualities (see 

 Fig. 138). White becomes black, red a bluish green, and vice versa. 

 Negative after images are produced very easily by fixing the eyes 



