318 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
B. OBSERVATIONS. 
1. SrEcTION 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 probability 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 31; the original inten- 
sity and for an exposure of fifteen minutes the efficiency of the red 
showed slight signs of weakening; and at és of the initial intensity, 
7. e. at 550 em. 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 th_ee 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 
