182 THE PERCEPTION OF LIGHT AND COLOR. 



varies in an entirely different manner ; it diminishes regularly from the 

 center to the periphery, where it has even appeared to certain experi- 

 menters to be entirely absent. 1 * * * The sensation of white, which 

 according- to Helrnholtz is the most complex sensation, is, on the con- 

 trary, the most simple and the one most easily produced. If any 

 simple spectral color be presented to the eye, the first sensation 

 produced (that which requires the least light for exciting the retina) 

 is a purely colorless sensation. To produce the idea of color it is neces- 

 sary to more strongly excite the retina, to present to it a much stronger 

 stimulus. * * * 



"Then, again, these two functions may vary independently of one 

 another, not only on account of their different localization, but also 

 because of the different influences exerted on them by certain physio- 

 logical conditions. Thus I showed in 1878 that the adaptation of the 

 eye to obscurity increases the luminous sensibility and hardly affects 

 the chromatic sensibility; hence the fact, inexplicable in the Helm- 

 holtz theory, that a simple color seen by an eye that has been kept in 

 darkness is perceived mixed with white. If this is taken together with 

 the fact that a pure color that appears saturated at the center of the 

 retina, appears more and more mixed with white (finally becoming 

 entirely white or gray) in proportion as it is viewed more indirectly, it 

 is easily seen with what facility white is produced by a physically 

 simple excitation. * * * 



" On December 27, 1880, I differentiated a new function of the 

 retina, independent of luminous sensibility and of chromatic sensi- 

 bility, namely, visual sensibility. I showed that the perception of a 

 group of small lumiuous points in central vision passes through two 

 phases, 2 precisely analogous to the two phases of color perception ,• the 

 one of indistinct vision (gross visual sensibility), the other of distinct 

 vision (visual sensibility, properly so called). The first requires for its 

 excitation less light than the second and may vary independently of 

 the other." 



After having alluded to the researches of M. Parinaud of 1891, M. 

 Charpentier continues as follows: 



"During these two years MM. Mace de Lepinay and Nicati published 

 their valuable researches on the distribution of brightness and of visual 

 definition in the spectrum, and discovered the important fact that 

 luminous intensity and visibility do not vary in the same ratio from 

 one color to another, the brightness relatively predominating in the 

 more refrangible part of the spectrum." 



These statements, which are taken from the article in the Revue 

 Generale, could be advantageously completed by the details given in 

 the treatise on light and color; but, for the sake of brevity, we will 

 simply give the results of the experiments made for the purpose of 

 determining the ratio between the intensity required for producing a 

 luminous sensation and a clearly distinguishable color sensation through- 

 out the different parts of the spectrum. 



1 CharpeuStier adds that this is erroneous. 



2 By direct vision, is understood vision by means of the fovea; and by indirect 

 vision, that by means of the eccentric parts of the retina. 



