342 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of the existence of light with special reference to its varying intensity, 

 and that the cones arc organs specially modified for the localization of 

 stimuli ami for the perception of differences of wave-lengths. The view 

 that the rods are specially adapted for the perception of luminosity and the 

 cones for that of color derives support from the fact that in the retina of cer- 

 tain nocturnal animals — e. g. bats and owls — rods alone are present. This 

 theory has been further developed by Von Kries, 1 who in a recent article 

 describes the rods as differing from the cones in the following respects: (1) 

 They are color-blind — i. c. they produce a sensation of simple luminosity 

 whatever be the wave-length of the light-ray falling on them ; (2) they are 

 more easily stimulated than the cones, and are particularly responsive to light- 

 waves of short wave-lengths; (3) they have the power of adapting themselves 

 to light of varying intensity. 



On this theory it is evident that we must get the sensation of white or 

 colorless light in two different ways : (1) In consequence of the stimulation 

 of the rods by any sort of light- rays, and (2) in consequence of the stimula- 

 tion of the cones by certain combinations of light-rays — i. e. complementary 

 colors. In this double mode of white perception lies perhaps the explanation 

 of the effect of varying intensity of illumination upon the results of color- 

 mixtures which has been above alluded to (see p. 337) as an objection to the 

 Hering theory. The so-ealled " Purkinje's phenomenon," described on p. 340, 

 is readily explained in accordance with this theory, for, owing to the greater 

 irritability of the rods, the importance of these organs, as compared with the 

 cones, in the production of the total visual sensation is greater with feeble 

 than with strong illumination of the field of vision. At the same time, the 

 power of the rods to respond particularly to light-rays of short wave-length 

 will cause a greater apparent intensity of the colors at the blue than at the red 

 end of the spectrum. In this connection it is interesting to note that the phe- 

 nomenon is said not to occur when the observation is limited to the fovea 

 centralis, where cones alone are found. 2 



Saturation. — The degree of saturation of light of a given color depends, as 

 above stated, upon the amount of white light mixed with it. The quality of 

 light thus designated is best studied and appreciated by means of experiments 

 with rotating disks. If, for instance, a disk consisting of a large white and a 

 small red sector be rapidly rotated, the effect produced is that of a pale pink 

 color. By gradually increasing the relative size of the red sector the pink 

 color becomes more and more saturated, and finally when the white sector is 

 reduced to zero the maximum of saturation is produced. It must be borne 

 in mind, however, that no pigments represent completely saturated colors. 

 Even the colors of the spectrum do not produce a sensation of absolute 

 saturation, for, whatever theory of color vision be adopted, it is evident that 

 all the color-perceiving elements of the retina are affected more or leas by all 

 the rays of light. Thus when rays of red light fall upon the retina they will 



1 Zeiischrift Jur Psychologic wnd Physiologic der Siwiesorgane, ix. 81. 



2 von Kries: CentralblaitfUr Physiologie, 1896, i. 



