DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 333 
turbed by differences in the rate at which the several colors diverged 
after issuing from the light-box, I measured the width of each band at 
two positions. The ratios which obtained between widths of the 
color-bands at one position were found to hold also at the other, so 
that the intensities were consequently always the same. 
The most important checks throughout the investigation have 
been:— the radiomicrometer for intensity; check animals for the 
series run by the section method; the unexposed eye of each animal 
for the series of direct observations; the verification of the records 
of direct observations by sections; and the photographic test for 
leakage of white light. 
B. SUMMARY OF OBSERVATIONS. 
1. Different regions of the spectrum at equal intensity elicited 
different amounts of pigment migration. 
2. Blue-violet was more potent than red, as evidenced both by 
sections and by direct observations of the crayfish eye. 
3. The quantitative expression for the difference in efficiency 
as determined by varying the intensity of the light was BV = 5.6 R; 
and 
4. As determined by varying the period of exposure, BV = 7.4 R. 
5. Blue-violet, green, and yellow ranked close together, but of 
the three blue-violet was probably the more efficient in evoking the 
migration. 
6. The rate of migration of pigment varied with the physiological 
condition, being slow in a feeble or sluggish animal and more rapid 
in a vigorous one. 
7. The quantitative expression for the efficiency of blue-violet 
in terms of that for red was probably a variable dependent on the 
physical condition of the animal. 
8. A bleaching of color from metallic orange to red and then to 
dull yellow was observed in the eye exposed to light. The possibility 
is that this phenomenon is due to a chromatic substance located in 
the rhabdomes, which undergoes a partial bleaching and is analogous 
to visual purple in the vertebrate retina. 
C. THe FuNcTION OF THE PIGMENT. 
The fact that the migration of pigment varies with the color of the 
light evoking it has an important bearing upon the problem of the 
function of the pigment. Hesse (:02) has reviewed the early concep- 
