THE PERCEPTION OF LIGHT AND COLOR. 181 



proverb, and how often have we not repeated : "It is too dark to dis- 

 tinguish colors." The poet Bacan, who had an affection for nature so 

 rare in his time, tells us furthermore in his Bergeries : 



"The shades of night with their own dusky hue 

 Alike the meadows and the fields imbue." 1 



Before taking up the study of Charpentier's researches we will first 

 consider the variation of chromatic sensibility in the different parts of 

 the retina. To quote M. Parinaud: "Chromatic sensibility decreases 

 from the center to the periphery, whether or not the retina be adapted 

 to obscurity. Besides, it decreases unequally for the different colors. 

 The radial distance at which a color ceases to be perceived varies 

 according to the intensity of illumination, the saturation of the color, 

 and in addition according to the extent of the colored surface. It is 

 therefore difficult to define the extent of the field of vision for each 

 color. The relations which these fields of vision bear to one another 

 are, on the contrary, quite fixed. The fundamental colors are lost to 

 perception in passing from the center to the periphery of the retina in 

 the following order : Green, red, yellow, blue. It may be said with truth 

 that the peripheral parts of the retina present a normal Daltonism; 

 indeed, in certain regions one can observe color-blindness for the green 

 and red, with normal vision for the yellow and blue, as in Daltonism." 2 



Mention must also be made of the faculty of visual definition — that 

 is to say, of the faculty of perceiving forms. This attains its greatest 

 perfection in the fovea, and decreases rapidly with the distance there- 

 from; but, as stated above, when the retina is strongly adapted the 

 fovea is relatively much less sensitive than the exterior parts of the 

 retina. The latter tend to supplement the central portions, and thus 

 the definition increases from the center to the periphery. 



Let us now study anew the same phenomena under the direction of 

 Oharpentier. There will necessarily be some repetition, but it may not 

 be without interest to clearly understand in what respects this investi- 

 gator agrees with the former and in what he differs. A resume of his 

 investigations has been given in the treatise already cited, " La Lumiere 

 et les Couleurs," and also in an article on " The origin and mechanism 

 of the different varieties of luminous sensations." :! 



" In exploring, by means of a special photometer and by the method 

 of minimum perceptibility, the sensitiveness of different parts of the 

 retina, I have shown, 7 ' says Charpentier, " that the acuteness of per- 

 ception of uncolored light (that is, white light, including the different 

 shades between white and black) is equally great on every part of 

 the retina except at the center, where it is least. Color sensibility 



1 C'est l'onibre de la nuit, dont la noire paleur 

 Peint les champs et les pres d'une meme couleur. 

 2 Page 71. 



"La Revue Ge"n6rale des Sciences Pures et Applique"es, July 15, 1898. The same 

 review had published M. Parinaud's article on the functions of the retina (April, 1898). 



