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 mm between them, 

 were used: — 



Blue-violet 430-490 mm 



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 

 flowers 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, 

 i. e. in a direction at right angles to the long axis of the light-box, 

 dintil 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 to 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 



