788 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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. 783) as an objection to the 

 Hering theory. The so-called " Purkinje's phenomenon," described on p. 787, 

 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. 1 



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 less by all 

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

 stimulate not only the red-perceiving elements, but to a slight extent also (to 

 use the language of the Helmholtz theory) the green- and violet-perceiving 

 elements of the retina. The effect of this will be that of mixing a small 

 amount of white with a large amount of red light i. e. it will produce the 

 sensation of incompletely saturated red light. This dilution of the sensation 

 can be avoided only by previously exhausting the blue- and green-perceiving 

 elements of the retina in a manner which will be explained in connection with 

 the phenomena of after-images. 



Retinal Stimulation. Whenever by a stimulus applied to an irritable 

 substance the potential energy there stored up is liberated the following phe- 

 nomena may be observed : 1. A so-called latent period of variable duration 

 during which no effects of stimulation are manifest ; 2. A very brief period 

 during which the effect of the stimulation reaches a maximum ; 3. A period 

 of continued stimulation during which the effect diminishes in consequence of 

 the using up of the substance containing the potential energy i. e. a period 

 1 Von Kries : Centralblatt fur Physiologic, 1896, i. 



