346 LESLIE B. AREY 



from such photomechanical changes, the responses in dim Ught 

 must be identical with those demonstrably occurring in the dark. 

 This assumption, however, is largely gratuitous. 



The supposition of an identity between the set of responses 

 ensuing in dim light and in darkness is further complicated by 

 certain conflicting statements and beliefs. There is a general 

 impression current that the retinal pigment and visual cells react 

 to the slightest traces of light; this is reflected in the statements 

 of many workers who have feared their results would be im- 

 paired unless the strictest precautions were observed. 



On the basis of actual experimentation, however, the earliest 

 observations are encountered in the writings of Angelucci ('90), 

 who reported that five minutes of candle light caused the frog's 

 cones to be highly shortened, whereas the pigment remained as 

 in darkness. On another page, nevertheless, he records that 

 after twenty minutes of twilight the pigment assumed the light 

 position, but the cones are influenced to a less degree! 



Somewhat later, Pergens ('97) wrote that after a five hours' 

 exposure to colored lights (red, yellow, green, and blue) of an 

 intensity such that colors could be distinguished by an observer 

 after one minute of dark adapta'tion, the cones of the white fish, 

 Leuciscus rutilis, are strongly retracted. The weakest response 

 was reported from the blue — a result, however, not in agreement 

 with Herzog's ('05) later findings on the frog. The latter 

 worker found the blue-violet most effective, although it should 

 be added that the duration of exposure employed by him was 

 only two minutes. 



In a further communication ('99) Pergens confirmed his earlier 

 conclusion that the pigment migrates least extensively in red 

 light (equal intensities being used), but mod fied his previous 

 belief regarding the inefficiency of the blue to provoke cone 

 retraction. 



Exner and Januschke ('05) performed some experiments, 

 which, unfortunately, are not trustworthy as evidence. Speci- 

 mens of the fish Abramis brama were exposed during the late 

 afternoon, the experiment continuing through the period of 

 failing light and terminating at dusk. Examination showed the 

 cones to be in the position characteristic of light. 



