day: PIGIIENT-MIGRATION IN EYE OF CRAYFISH. 343 



Explanation of Plates. 



Plate 1 exhibits the typical extreme condition for an e3'e adapted to light 

 (Fig. 1), and for one adapted to darkness (Fig. 2), both as seen bj^ direct obser- 

 vation (Figs, la, 2a) and in a section of the eye (Figs, lb, 2b). 



Plates 2 and 3 show sections of four eyes exposed respectively to blue-violet, 

 green, yellow, and red of equal intensities and for the same length of time. 

 Evaluations for the relative amounts of migration are given in Table I, series 3. 



Plate 4 and Fig. 9, Plate 5, are checks on the migration of pigment as re- 

 corded by direct observation (a), and as revealed by sections of the same eyes 

 (b). 



Plate 5, Fig. 10 is a photographic test, which gives evidence of no error due to 

 leakage of white light from the light-box. 



Figs, la, 2a, 7a, 8a and 9a are copies of the original records. The magnifica- 

 tion is about 10 diam. The orange (metallic) glow of the eye (Fig. 2a) was 

 due to light reflected by the tapetum back through the orange-colored rhab- 

 domes. Bleaching to a dull yellow occurred in the light (Fig. 9a). Con- 

 comitant with this a dark area appeared, which was an index of the amount of 

 pigment-migration: in Fig. 2a there is none; Figs. 7a, 8a and 9a show succes- 

 sive steps of the change induced by Ught, which culminates in Fig. la. The 

 appearance of the dark area in only the upper half of Figs. 7a, 8a, and 9a was. 

 probably an optical effect due to refraction in the cones. 



The figures of all the sections, which were median longitudinal ones cut in 

 an antero-posterior direction, are photographic reproductions made with a 

 combination of Leitz oc. 4 and Zeiss a* obj. upon a Seed's 26 plate and magni- 

 fied about 40 diam. 



