MOVEMENTS IN THE VISUAL CELLS 129 



ness, maximal light- and dark-adaption of the pigment respec- 

 tively was produced. Kiihne (79) stated that complete dark- 

 adaption occurred in 1 to 2 hours, this period corresponding 

 to the time necessary for the regeneration of the visual purple. 

 Chiarini f'04b) maintained that following an exposure of half 

 an hour to direct sunlight one and one-half hours were needed 

 to complete the adaption in darkness. Although no direct 

 experimentation was made on the frog, the general experience 

 gained from working with this animal leads me to suspect that 

 1 hour for light-adaption and 1 to 2 hours for dark-adaption 

 are approximately the proper amounts. 



h. Visual cells 



Relative to the adaption times of the cone cells, Pergens 

 ('96) stated that in Leuciscus the first visible shortening occurred 

 after an exposure to light of 1 minute. According to his illus- 

 trations, the cones were very much shortened after 2 minutes 

 while those that had been subjected to light for 5 minutes were 

 practically in the position characteristic for light. When light - 

 adapted animals were introduced into the dark, a lengthening 

 of the cones was evident after 1 minute and in 5 minutes the 

 elongation was nearh^ complete, although it did not become 

 maximal for 20 minutes. Pergens' method of allowing decapi- 

 tated heads to remain in the light during fixation has been criti- 

 cized by Garten ('07), and the later work of Pergens ('99) has 

 likewise been questioned by Herzog ('05). These workers assert 

 that it is entirely possible that the changes initiated by the action 

 of light continue until the actual penetration of the fluid into 

 the eye fixes the retina. Certain experiments of \\'eiss, insti- 

 gated by Garten, indicate that the action of light can influence 

 the position of cones and retinal pigment after decapitated heads 

 have been introduced into the fixitive. These results are in 

 opposition to the statement of Ghiarini ('04b) that light can- 

 not be effective during fixation. 



In an attempt to avoid this source of error, at the completion 

 of an experiment the hght-adapted eyes were fixed in exceedingly 



