318 bulletin: museum of comparative zoology. 



B. Observations, 



1. Section method. A series of preliminary tests was made to 

 ascertain the time required for the pigment to attain the complete 

 recessive condition for dark-adaptation. The probabiHty is that it 

 varies with the physiological state of the animal, being more rapid 

 for a vigorous individual than for one in poor condition. Although 

 four hours were found to suffice, yet, for the sake of certainty, the 

 period allotted for dark-adaptation was six hours. 



In the experiments with colored lights two methods were em- 

 ployed: — the first consisted of exposing the crayfish to the color, 

 killing it instantly in hot water and preparing the eye for microscopic 

 study; while by the second a comparison of the influence of the 

 different colored lights in extinguishing the glow of the living eye was 

 made by direct observation. 



(a). Procedure. In employing the first method the animals were 

 exposed at different distances from the source of light and for different 

 periods of time. The crayfish was submerged in a tank of water just 

 far enough to permit respiration but not to cover the eye during the ex- 

 posure (Fig. F.). At first the animals were held by a clamping device, 

 but later by hand. When a strong intensity and long exposure (one 

 hour) were tried, sections of the eyes revealed no detectable difference 

 in the influence of the several colors because the amount of migration 

 was the same in every case. The intensity was then diminished by 

 moving farther away from the light, and simultaneously the length 

 of time for the exposure was. also reduced. At 2V the original inten- 

 sity and for an exposure of fifteen minutes the eflficiency of the red 

 showed slight signs of weakening; and at r.h-^ of the initial intensity, 

 i. e. at 550 cm. distance, or as far away from the light as the crayfish 

 could be placed, the red was decidedly behind the other colors in 

 effect, while the blue-violet, green and yellow remained about equal. 

 To differentiate the latter was the next problem. The exposure was 

 cut down to one minute. A stop-watch together with precautions for 

 eliminating delay between the end of the exposure and the immediate 

 fixation of the pigment in situ, refined the process to the desired 

 degree. Although the three colors could not all be separated, never- 

 theless the efficiency of yellow and green, as yet on a par with each 

 other, could be differentiated from that of the more potent blue- 

 violet. 



(b). Results. The results obtained by the section-method are 

 given in Table I. The nine series, each including usually four 

 animals, were run upon separate days. In each series a single 



