PROPERTIES OF THE RETINA. 347 



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. 



In accordance with these definitions our color sensations may 

 be said to have three characteristics (Priest), namely, brightness, 

 hue, and saturation. The brightness is determined by the amount 

 of the stimulus, the hue and saturation by the spectral distribution 

 of the stimulus. 



Color Fvsion. — 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 tning 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 simultaneousl}^ 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 

 being a system of lenses or mirrors by which different portions of 

 a spectrum can be superposed. The usual device employed in 

 laboratory experiments is that of rotation of discs of colored paper. 

 Each disc has a sHt in it from center to periphery so that two discs 

 can be fitted together to expose more or less of each color. If a 

 combination of tliis kind is attached to a small electrical motor it 

 can be rotated so rapidly that the impressions of the two colors 

 upon the retina foUow at such a short interval of time as to be prac- 

 tically simultaneous. 



The Fundamental Colors. — By the methods of color fusion 

 it can be shown that three colors may be selected from the spec- 

 trum whose combinations in different proportions will give white, 



