SENSATIONS OF COLOR 541 



3. SENSATIONS OF COLOR 



There are three different modifications of light which influence our sensa- 

 tions of color : brightness, which depends upon the energy of the ether vibra- 

 tions; tone, which depends upon the wave length; and saturation, which de- 

 pends upon the purity of a given wave length, or, in other words, upon the 

 amount of white light present. 



The human eye can distinguish all of these properties. Indeed, its ability 

 to distinguish differences of color is very highly developed. Konig has esti- 

 mated that in the visible spectrum there are 165 different color tones, which 

 can be distinguished, and that the total number of different degrees of in- 

 tensity perceptible to the human eye is about 660. When we remember that 

 each tone can vary greatly in intensity and each tone and intensity in turn 

 can have all possible degrees of saturation, we get some idea of the number 

 of possible color sensations. 



According to Herschel the mosaic workers in the Vatican can distinguish 

 30,000 different colors. 



A. RELATION OF THE PROPERTIES OF LIGHT TO DIFFERENT 

 CONSTITUENTS OF THE RETINA 



Some color tones of the spectrum always appear brighter than others: 

 with light of ordinary intensity the brightest of all is a certain tone of yellow 

 (535^,). If the different colors be observed as the daylight fades it will 

 be noticed that certain ones disappear sooner than others e. g., the reds 

 before the blues. In a very feeble light a weak spectrum can still be seen, 

 but only as a band of light. Its colors have disappeared and that part of 

 the spectrum which now appears brightest is nearer the more refractive end 

 of the spectrum than it was in broad daylight. But if only a portion of the 

 weak spectrum small enough to be imaged on the fovea centralis be allowed 

 to enter the eye, its color can be correctly perceived (Konig, Sherman). 



These and other facts have led to the assumption of a functional differ- 

 ence between the rods and cones. The latter, found everywhere in the retina 

 and exclusively in the fovea centralis, are thought to be sensitive to light of 

 different wave lengths, but to require rather a high degree of illumination. 

 The rods are sensitive to a much feebler light but are not sensitive to color 

 tones (v. Kries, Parinaud et al.). Eyes which are totally color blind, remain- 

 ing sensitive only to light and darkness, are therefore supposed to be devoid 

 of cones. Nocturnal animals like the owl, bat, mouse, cat, etc., are known to 

 have relatively fewer cones and more rods than diurnal animals (Max 

 Schultze). 



The visual purple (cf. page 537) is also thought to assist in vision by a 

 feeble light. In the first place, it is not found in the cones; and, in the 

 second, there is a close agreement between the brightness of the different 

 wave lengths in a feeble light and their action upon solutions of the visual 

 purple. Accordingly it is conjectured that the fading of the visual purple 

 is of some service in the stimulation of the rods. 



