350 THE SPECIAL SENSES. 



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 

 in 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 

 or more color processes in the retina, this end being obtained by 

 superposing upon the same portion of the retina the rays giving 

 rise to these color processes. It must be borne in mind that color 

 fusion upon the retina is quite a different thing from color mixture 

 as practised by the artist. A blue pigment, such as Prussian blue, 

 for instance, owes its blue color to the fact that when sunlight falls 

 upon it the red-yellow rays are absorbed and only the blue, with 

 some of the green, rays are reflected to the eye. So a yellow pig- 

 ment, chrome yellow, absorbs the blue, violet, and red rays and 

 reflects to the eye only the yellow with some of the green rays. A 

 mixture of the two upon the palette will absorb all the rays except 

 the green and will therefore appear green to the eye. If, however, 

 by means of a suitable device, we throw simultaneously upon the 

 retina a blue and a yellow light, the result of the retinal fusion is 

 a sensation of white. Many different methods have been employed 

 to throw colors simultaneously upon the retina, the most perfect 



