PROPERTIES OF THE RETINA. 331 



esses causing white and black. Referring only to the colors proper, 

 the fundamental color sensations according to some views are red, 

 green, and blue or violet; according to others, they are red, yellow, 

 green, and blue. (See paragraph on Theories of Color Vision.) 



Helmholtz calls attention to the fact that the names used for these funda- 

 mental color sensations are obviously of ancient origin, thus indicating that 

 the difference in quality of the sensations has been long recognized. Red is 

 from the Sanskrit rudhira, blood; blue from the same root as blow, and re- 

 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 



