180 THE PERCEPTION OF LIGHT AND COLOR. 



Let us begin with a study of the psychic phenomena which have been 

 observed — that is to say, the sensations and perceptions; next we will 

 consider the physiological phenomena to which these psychic phe- 

 nomena appear related, and finally the theories which have been erected 

 on the framework of this ensemble of facts will be briefly reviewed. 



I. — PSYCHIC PHENOMENA. 



The phenomena which we are about to describe were first discussed 

 in M. Charpentier's publications, as has already been stated, but the 

 results reached by him are of a more complicated character than those 

 of M. Parinaud, whose first contribution on the subject was published 

 in 1881, while that of his rival dates back to 1877. It is as if Begnault's 

 work had preceded that of Mariotte. Without pretending to decide 

 their dispute in regard to priority, let us remark that it is easier to 

 begin by a study of Mariotte, or in the present case of Parinaud. The 

 latter, in a recently published treatise, summarizes the results of his 

 experiments on the adaptation of the retina to obscurity as follows: 



1. The increase in the sensitiveness of the retina, which characterizes 

 adaptation to obscurity, varies according to the wave length of the 

 light ; it is greater the smaller the wave length. The influence of adap- 

 tation which is zero for spectral red becomes considerable for the violet 

 and ultraviolet. 



2. This increase of sensitiveness affects only the luminous value of 

 simple light. The color appears brighter and less saturated. Finally, 

 after a sufficient time spent in darkness, the purest spectral colors of 

 feeble intensity are perceived as uncolored light, the red alone excepted. 



3. This increase in the sensitiveness is lacking in the fovea. 1 The 

 fovea does not participate in retinal adaptation. The sensation of color 

 not being altered by adaptation in the fovea, luminous impressions are 

 always perceived in that region as colors. 2 



On reading these enunciations one is struck by the very characteristic 

 fact that although white has generally been considered as being essen- 

 tially a complex color, we see that for luminous excitations received 

 outside the fovea all light, the red excepted, is perceived first as color- 

 less and, for a retina adapted to obscurity, remains strongly diluted in 

 appearance, whatever be its intensity. There are, then, two kinds of 

 sensibility which, according to M. Charpentier, may be distinguished 

 by the terms luminous sensibility (properly so called) and chromatic 

 sensibility 3 . It may be remarked that, although disregarded in the old 

 scientific literature, these two kinds of sensibility have been long 

 familiar to ordinary observation. " At night all cats are gray," says the 



1 This is the name of the small hollow which exists in the middle of the yellow 

 spot and on which the image is focused in central vision. 



2 La Vision, Etude physiologique, 8vo., 218 pp., Octave Doin, 1898, pp. 47-48. 



? ' La Lumicre et les Couleurs au point de vue physiologique, vol. 1, 8vo, 352 pp., 

 Bibliotheque scientifique contemporaine, Bailliere et fils, 1888. See p. 209. 



