day: pigment-migration in eye of crayfish. 333 



turbed by differences in the rate at which the several colors diverged 

 .after issuing from the hght-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- 



