THE PERCEPTION OF LIGHT AND COLOR. 185 



it remains to be shown how they account for certain well-known facts 

 which have remained isolated and have not appeared to be logically 

 connected with the rest. 



In 1825 Purkinje pointed out the change which a color undergoes 

 if its intensity be gradually diminished; for example, if we take two 

 pieces of colored paper, the one red and the other blue, the latter of 

 which, under a moderate illumination, appears darker than the former, 

 and gradually decrease the illumination of the chamber in which 

 they are placed, the blue paper will gradually be seen to become 

 relatively lighter than the red. Indeed it is possible, by still further 

 diminishing the intensity of illumination, to reach a stage when the 

 blue paper will appear whitish gray and the red paper entirely black. 

 This is called the Purkinje phenomenon. Parinaud, in a resume of his 

 previous studies in his treatise on vision, 1 explains the phenomenon in 

 this manner: "It is not the differences of intensity of the colors that 

 produces the phenomenon, but differences in the illumination of the. 

 retina which observes them. The phenomenon is due, not to an objec- 

 tive, but to a subjective cause, and finds its natural explanation in the 

 properties of the retina. It results from the unequal influence of the 

 adaptation of the retina to rays of different refrangibility, and also 

 from the fact that adaptation only affects the luminous value of colors 

 and not the color sensation itself." 



Hering, in an article published in 1895, arrived at analogous conclu- 

 sions. He discovered, in fact, that the diminution in the intensity of 

 colors alone is not sufficient to produce the Purkinje phenomenon; this 

 manifests itself if the room in which the subject is placed be darkened, 

 and is the more pronounced the more perfectly the eye is adapted to 

 obscurity. It is more characteristic of indirect than of direct vision. 



In 1896 Von Kries showed that the Purkinje phenomenon is absent in 

 direct vision, and for indirect vision its intensity is proportional to the 

 degree of adaptation of the eye to darkness. 



All these results, according to Parinaud, appear fully explained by 

 the fundamental phenomena discovered by himself and Charpentier. 

 The latter, however, explains the results in an entirely different man- 

 ner. By observing for each spectral color the additional illumination 

 which is necessary to enable the eye to distinguish the difference 

 between the new and the previous illumination, he deduces a curve 

 showing the relation between the intensity of the sensation and the 

 intensity of its stimulus, according to Fechner's principle, without, 

 however, being able to verify the law which bears the latter's name. 

 He has thus established that the less refrangible colors gain much 

 more in brightness than the more refrangible ones when the intensity 

 of the light is continuously increased. 2 Prom this observation the Pur- 

 kinje phenomenon can be deduced. It will also be remarked that 



*Op. cit., p. 67. 



2 La Lumiere et lea Couleurs, p. 333. 



