DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 323 
position as to expose one eye only to the direct action of the light, and 
with two or three diaphragms interposed between it and the source 
of light to exclude all reflected rays. At the close of the exposure, 
which was timed by a stop-watch, the eye was examined by focusing 
a flash-light on it, a judgment was made by direct observation of the 
amount of darkening in its center, and the condition was sketched 
into a standard blank circle representing the eye, stamped in the 
record book. The coloration of the eye was also indicated in the 
circle by means of colored pencils. The eye on the side of the head 
away from the light, since it retained its glow and showed no dark 
center (when the exposure did not exceed five minutes), served as a 
check upon the other and was recorded in like manner. 
(c) Preliminary tests. The first step, as in the section method, was 
to make preliminary tests for differences in the effects of the three 
chosen spectral regions, blue-violet (430-490 wy), yellow-green (524— 
576 wu), and red (625-665 wu). As before, many trials were necessary 
to find the most favorable combination of intensity and exposure to 
elicit perceptible differences in the external appearance of the eye 
correlated with the migration of pigment. Whereas in the spring, 
when the first method had been applied, a weak intensity and short 
exposure had yielded the desired results, in the winter, when the 
second method was employed, the animals, being in a more sluggish 
condition, required a much higher intensity and longer exposure. 
The outcome of the preliminary observations confirmed those obtained 
from the sections: — the blue-violet was decidedly more potent then 
the red, but the case of blue-violet vs. yellow-green was doubtful, 
since they were so close in efficiency. 
(d) Effects of blue-violet and red compared. The method was then 
concentrated upon the question of obtaining a quantitative expression 
for the greater efficiency of the blue-violet as compared with red: 
first, by varying the intensity while keeping the time of exposure 
constant, and secondly, by varying the time while keeping the intensity 
constant. 
(1) Intensity varied. Ten minute exposures were made to blue- 
violet at a distance of 200, 250, 300, and 400 em., and to red at 50, 100, 
150, and 200 cm., with the expectation that positions giving equiva- 
lent migration in the blue-violet and the red might be obtained. 
These distances were measured from a point situated several centi- 
meters in front of the light-box and corresponding to the position of 
the radiomicrometer when the lights had been balanced in intensity. 
It was found that the full number of exposures could not be made on 
