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. MeEtTHODS 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 
