194 THE PERCEPTION OF LIGHT AND COLOR. 



Charpentier lias shown that from two or three times to eighteen or 

 twenty times more light is needed for distinguishing the luminous 

 points than for merely apprehending the primitive luminous sensation. 

 The difference is still greater for an adapted eye, as we might have fore- 

 seen, since it is known that adaptation does not develop the sensitive- 

 ness of the yellow spot, where visual perception is at its best. Continu- 

 ing the study of the phenomenon, if the small points are successively 

 illuminated with the different spectral colors, and if the ratios of the 

 intensity required for the luminous sensation and that permitting a 

 distinction of the points be determined, it is observed that these ratios 

 exceed unity in proportion as the color is more refrangible. In this 

 connection we may recall analogous facts relative to the distinction 

 between luminous sensibility and color perception. This general agree- 

 ment, expressed in exact figures, is transformed into an almost rigor- 

 ous proportionality, for the ratio between the intensities required 

 for distinct vision of the points and for the perception of their 

 color only varies between 1.80 and 1.93, according to the spectral color 

 employed. 



From these facts Parinaud concluded that the retinal elements 

 involved in distinct vision are the same as those involved in color per- 

 ception, to which conclusion, however, Charpentier takes exception. 

 The latter points out that the distribution of visual sensibility through- 

 out the extent of the retina is not the same as that of color perception, 

 which decreases regularly from the center of the retina to its periphery, 

 while visual sensibility decreases much more rapidly. On the other 

 hand, in the fovea itself the perception of colors is almost zero, while 

 the visual sensibility has there its maximum. 3 If it be remarked 

 that luminous sensibility is also very feeble in the fovea, one is led to 

 the conclusion that chromatic sensibility exists only in those regions 

 where both luminous and visual sensibilities exist at the same time, 

 and moreover that it undergoes variations throughout the extent of 

 the retina, which correspond sufficiently well to the mean of these two 

 functions. 



One is thus led to the view that color sensation is due to the combined 

 action of the elements of both the luminous and visual sensibilities. It 

 is thus induced by a physiological fact, due to the simultaneous exist- 

 ence of two distinct impressions produced in the organ of sight by 

 luminous rays. Starting out from this conception, naturally suggested 

 by the facts, Charpentier formulates a bold theory of which he is the 

 first to recognize the highly hypothetical character, but which, it 

 appears to us, deserves a detailed analysis. 



In seeking the possible nature of the two fundamental actions we are 

 led to the view that the former or photoa^sthetic action (being photo- 

 chemical) is due to the visual purple. The second or visual action 



1 Corresponding with this fact, we find that the nerve cells in relation with the 

 macula are bipolar and not multipolar, as elsewhere in the retina. 



