THE RETINA 843 



solutions thus obtained, contain the visual purple in its original form 

 and may be bleached by exposing them to light. It does not seem 

 likely, however, that this reduction gives rise to distinct bodies, such 

 as have been designated by Kiihne as visual yellow and visual white. ^ 

 A dissolution of this pigment results in alkalies, alcohol, ether, chloro- 

 form and most acids. It is resistant against ammonia, sodium chlorid, 

 benzol, fats and oils. Even the different rays of the spectrum affect 

 it in an unequal measure, red and orange being least destructive 

 and yellow and green most destructive. 



The Function of the Visual Purple. — While it is perfectly obvious 

 that the visual purple is an unstable pigment which is decomposed by 

 the ethereal impacts, this fact does not furnish an adequate explana- 

 tion for the changes resulting in the rods and cones in consequence of 

 the vibratory energy imparted to them by the ether waves. Neither 

 is it possible to. recognize in this pigment a substance which is abso- 

 lutely essential to vision, because it is absent in some animals, such as 

 the pigeon, hen, certain reptiles, and bats, and remains wholly confined 

 to the rods. Consequently, since the fovea centralis is composed 

 exclusively of cones, it is absent from this area which, admittedly, 

 is the place of most acute vision. 



These discrepancies force us to assume that the visual purple 

 serves merely as a sensitizing substance which is made use of chiefly 

 in low intensities of light. It is a well-known fact that the sensitiveness 

 of the fovea decreases in dim light, while that of the peripheral expanse 

 of the retina increases. In other words, while the cones are employed 

 in day-vision, the rods are brought into more general use in low inten- 

 sities of light. In semi-darkness, therefore, we invariably endeavor 

 to bring the image into the peripheral retinal field by slightly diverging 

 the eyes, while in daytime we focalize the object direptly upon the 

 yellow spot. This shows first of all that the cones themselves are 

 sensitive to light and need no sensitizing substance in ordinary light. 

 Their acuity, however, decreases steadily with the intensity of the 

 light, just because they are devoid of this pigment. For this reason, 

 therefore, the yellow spot becomes practically blind in semi-darkness. 

 By analogy, it may then be concluded that the greater sensitiveness 

 of the peripheral zone of the retina in the dark-adapted eye is directly 

 dependent upon the production of the visual purple and its movement 

 to the outer segments of the rods. By virtue of this pigment, these 

 elements are enabled to raise the otherwise inert light rays above the 

 threshold of stimulation. In this connection, brief reference should 

 also be made to the view of von Kries,^ according to which the percep- 

 tion of color is distinctly a function of the cones, while the rods are 

 regarded merely as playing a- part in the perception of white light of 



1 Abellsdorfif and Kottgen, Zeitschr. fiir Psychol, und Physiol, der Sinnesorgane, 

 xii, 1896. 



2 Zeitschr. fiir Phychol. und Physiol, der Sinnesorgane, ix, 1895, 81. 



