day: pigment-migration in eye of crayfish. 323 



position as to expose one eye only to the direct action of the Hght, and 

 with two or three diaphragms interposed between it and the source 

 of hght 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 daikening 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 /x/z), yellow-green (524- 

 576 /i/x), and red (625-665 mm)- 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 cm., 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 



