PROPERTIES OF THE RETINA. 321 



combined action of certain pairs of colors, complementary colors, 

 as is described below. Black, on the contrary, is the sensation 

 caused by withdrawal of light. It must be emphasized that in 

 order to see black a retina must be present. It is probable that 

 a person with both eyes enucleated has no sensation of darkness. 

 That black is a sensation referable to a condition of the retina is 

 made probable also by the interesting observations recorded by 

 Gotch,* namely, that when an eye that has been exposed to light 

 is suddenly cut off from the light there is an electrical change in the 

 retina, a dark response, similar to that caused by throwing light on 

 a retina previously kept in the dark. Blackness, therefore, is a 

 sensation produced by withdrawing light from the retina, and a 

 black object is one that reflects no light to the eye. Black may be 

 combined with white to produce the series of grays, and when com- 

 bined with the spectral colors it gives a series of modified color tones, 

 thus the olives of different shades may be considered as combina- 

 tions of green and black in varying proportions. 



The chromatic series consists of those qualities to which we give 

 the name of colors, and, as stated above, they compose the spectral 

 colors, and the extraspectral color, purple, together with the light- 

 weak and light-strong hues obtained by combining the colors with 

 white or black. In the spectrum many different colors may be 

 detected, some observers record as many as one hundred and 

 sixty, but in general we give specific names only to those that 

 stand sufficiently far apart to represent quite distinct sensations, 

 namely, the red, orange, yellow, green, blue, and violet. When 

 light is taken from a definite limited portion of the spectrum we 

 have a monochromatic light that gives us a distinct color sensation 

 varying with the wave length of the portion chosen. 



Color Saturation and Color Fusion. The term saturation as 

 applied to colors is meant to define their freedom from accompany- 

 ing white sensation. A perfectly saturated color would be one 

 entirely free from mixture with white. On the objective side it is 

 easy to select a monochromatic bundle of rays from the spectrum 

 without admixture of white light, but on the physiological side it is 

 not probable that the color sensation thus produced is entirely free 

 from white sensation, since the monochromatic rays may initiate 

 ir* the retina not only the specific processes underlying the pro- 

 duction of its special color, but at the same time give rise in some 

 degree to the processes causing white sensations. Even the spectral 

 colors are therefore not entirely saturated, but they come as near 

 to giving us this condition as we can get without changing the state 

 of the retina itself by previous stimulation. 



Color Fusion. By color fusion we mean the combination of two 



* Gotch, "Journal of Physiology," 29, 388, 1903. 

 21 



