day: pigment-migration in eye of crayfish. 329 



minutes, the latter is less than the former, indicating the possibility 

 that diminution of intensity is not compensated for by a correspond- 

 ing increase in the length of exposure. In Table V are given a few 

 exposures to yellow-green, which, when taken with the evidence 

 furnished by the section-method, indicate that the efficiency of this 

 region of the spectrum is not greater than that of the blue-violet and 

 that, although they rank close together, it is probably less. 



IV. Discussion. 



A. Methods and Checks employed. 



A comparison of the two methods employed in the foregoing in- 

 vestigation will show how they supplemented and confirmed each 

 other. 



The first procedure, by which the eye was sectioned and studied 

 microscopically, had various limitations as compared with the second : 

 — the delay necessitated by the removal of the cuticula; the use of 

 only one color for each animal; and ignorance of the initial position 

 of the pigment. From the last named limitation as a premise, it 

 could be argued that the final differences obtained in the effects of the 

 various colors might be due to initial differences in the position of the 

 pigment. At the season of the year, however, when the section 

 method was applied, viz. during May and June, the crayfish were in 

 vigorous condition, and since animals of the same sex and as nearly 

 the same size as possible were selected for a given series, and since 

 the check eyes, not only of Table I but also of earlier preliminary 

 tests, gave satisfactory evidence that the six hours allowed for dark- 

 adaptation was sufficient, the error from that source was probably 

 very slight. On the other hand, sections offered greater precision for 

 the determination of the photokinetic effects, in that they showed 

 the actual distance traversed by the pigment. This was not a uni- 

 form amount over the whole retina. The maximum migration 

 occurred at the center, caused probably by the concentration of light 

 in the vertical image of the Nernst filaments formed on the retina, 

 while on either side the amount gradually diminished. This diffusion 

 effect is shown in Plate 3, Fig. 6. In the left half of the photograph 

 the emigrated pigment has been indicated, by retouching the photo- 

 graphic print •with ink, as a darker band in order to distinguish it from 

 the tapetum, which also photographed dark. At the center they 

 practically  coincide in extent, but at the extreme left the pigment 



