DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 315 
but for the later experiments three regions, which centered upon 460, 
550, and 645 approximately, making intervals of 90 uu between them, 
were used :— 
Blue-violet 430-490 uu 
Yellow-green 524-576 “ 
Red 625-665: “ 
If all factors concerned, the lamp, lens, prism, and diaphragms had 
retained a constant position in the apparatus, then the different 
regions of the spectrum would always have coincided with the slots 
in their appropriate diaphragms; but, owing to slight shifting of the 
glowers due to expansion and contraction, it was found necessary to 
introduce an adjusting screw, s, Fig. C, by the manipulation of which 
from outside the box, the whole lamp could be moved laterally, 
2. e. in a direction at right angles to the long axis of the light-box, 
until the exact spectral region, as determined by the spectroscope, 
was obtained through the slot in the diaphragm. Prior to the em- 
ployment of each color the light was thus set by means of the adjust- 
ing screw and spectroscope. 
The only other pieces of apparatus required were vessels in which 
the animals could be kept in absolute darkness for several hours, 
and a few diaphragms which could be interposed at intervals between 
the animal and the front of the light-box. Tin troughs about three 
and a half feet long, eight inches wide and four inches deep, when 
coated with black wax on the inside and provided with light-tight 
covers served the first purpose; while screens of opaque cardboard 
served to eliminate any extraneous reflected light during the exposure 
of the animal to the monochromatic light. 
2. ANATOMICAL ORIENTATION. The animal used in all of my 
experiments was a common species of crayfish, Cambarus affinis. 
There were several advantages which, in comparison with the verte- 
brates commonly used, made this animal a favorable subject for 
experimentation; although not indigenous in the neighborhood of 
Cambridge, it easily adapted itself te the aquarium after importation 
from the South; its size was convenient for manipulation; the migra- 
tion of pigment was pronounced, and especially was it superior for 
the ease with which the eyes could be instantly fixed after exposing 
them to the colored light. 
A brief survey of the topography of the compound eye will be 
serviceable with respect to the problem in hand. For a detailed 
account of the anatomy one may turn to the description of the Euro- 
pean species, Astacus, by Parker (’95). The structure of the eye of 
