188 THE PERCEPTION OF LIGHT AND COLOR. 



to the discovery of Boll, Helinholtz had discovered the phenomenon of 

 fluorescence in the retina; Ewald and Kiihne established the fact that 

 this property belongs only to those parts of the retina which contain 

 the purple. In order to verify this in a marked manner it is necessary, 

 however, to experiment with an unbleached retina from the living 

 subject. A retina saturated with purple gives rise to a whitish fluor- 

 escence, which changes to greenish as soon as the retinal yellow 

 predominates, and finally becomes green for a decolorized retina. 

 These observations, of a physical character, bring us at ouce to a con- 

 sideration of the principal hypotheses which have been suggested to 

 connect them with the phenomena of sensibility discussed in Part I. 

 The theories (properly so called) which attempt to explain in a system- 

 atic manner the origin of color sensation will be reserved for Part III. 



From 1881 to 1885 Parinaud published a series of studies in which he 

 emphasized the separate and distinct roles played by the rods and the 

 cones in vision. As stated above, neither rods nor visual purple are 

 found in the fovea of which the sensitiveness is not at all increased by 

 darkness; and, moreover, since darkness influences only the luminous 

 sensation and not the chromatic sensation, we may conclude that the 

 cones are the organs of color sensation, while the rods and the visual 

 purple have nothing to do with it. Parinaud finds a confirmation of 

 this opinion in persons afflicted with hemeralopia; that is to say, inca- 

 pable of seeing in a dim light. In these the functions of the fovea, 

 where there is no purple, are intact; and on the other hand, hemeralo- 

 pia carries with it no deterioration of color vision. Moreover, in Dal- 

 tonism the luminous value of the colors not perceived is unaffected, at 

 least if the retina be adapted to obscurity. 



Later, in 1894, Parinaud emphasized the role which the fluorescence 

 of the retina plays in luminous sensation. Helinholtz denied this 

 explanation, basing his argument on its greenish coloration. This 

 coloration is, moreover, not essential, as we have seen, and there can, 

 moreover, be no necessary correlation between the objective properties 

 of the luminous agent and the sensation which it provokes; however, it 

 does not seem to us that Parinaud gives any proof of his own hypothe- 

 sis, since, as fluorescence constantly accompanies the purple and never 

 exists without it, it therefore appears impossible to separate its influ- 

 ence from that of the other properties of the purple. Nevertheless, 

 it should be pointed out that the sensation provoked by the blue, violet, 

 and ultraviolet radiations on a retina previously subjected to darkness 

 presents the special characteristics of sensations produced by fluo- 

 rescing bodies, and that the highly refrangible radiations are at the 

 same time those which affect the purple and produce the phenomena of 

 fluorescence. 



However this maybe, Parinaud admits that there is a certain difference 

 between the action of visual purple and the fluorescence of inorganic 

 substances. The latter does not appear to be accompanied by any dis- 



