day: pigment-migration in eye of crayfish. 321 



As regards blue-violet, green, and yellow, what evidence there is 

 points to the fact that they stand close together, but of the three 

 blue-violet is possibly the more efficient in eliciting tht migration of 

 pigment. 



Having discovered a d cided difference in the efficient*}' of blue- 

 violet and red, it was suggested that some quantitative expressiou 

 be sought for that difference: — how much more potent was the blue- 

 violet than the red? The attempts to obtain an answer by the section- 

 method are shown in the variations of the exposures to red in series 7 

 and 8; but they are too few to be even indicative. This question was 

 investigated and carried through to a conclusion by the second method 

 employed in the experiments, viz. by direct observation of the changes 

 in the glow induced by red and by blue-violet light, respectively, in 

 the dark-adapted eye. 



2. Direct-observation method, (a) Phenomenon of glow. The 

 fact that the phenomenon of glow had been observed by Lowne ('84) 

 in moths and butterflies, by Exner ('91) in various insects and Crus- 

 tacea, and by Kiesel ('94) in a moth, led me to investigate the matter 

 in the crayfish. In this animal, too, it was found that the dark- 

 adapted eye was quite different in aspect from the one adapted to 

 light. In the daytime the eye (Plate 1, Fig. la) presented a dark cen- 

 tral spot framed by a lighter peripheral area. As early as 1855 this 

 phenomenon had been described in Limulus by Leydig and the term 

 "Pseudopupille" applied to it from its apparent analogy to the pupil 

 in the vertebrate eye. Exner ('91) has investigated the phenomenon 

 among many arthropods and has offered an explanation for it in- 

 volving the principle of the cylinder lens, realized in the cones, and 

 also the role played by the pigment and tapetal cells. 



At night the pseudopupilla in the crayfish had vanished and the eye, 

 when examined by a flash-light, appeared no longer dark, but glowed 

 with a bright, metallic-orange light (Plate 1, Fig. 2a); and when 

 illuminated in the aquarium, the eyes of the crayfish shone out of the 

 darkness like beads of fire. It was not a case of fluorescence but of 

 reflection. Since in the dark-eye the pigment of the retinal cells 

 occupied a position proxima' to the basement-membrane (Plate 1, 

 Fig. 2b), the retinal tapetum was left exposed, and it acted as a reflec- 

 tor for all light entering the eye. The rudy orange hue was due to the 

 presence of a red substance in the rhabdomes,— observed by Lowne 

 ('84) in the cabbage-butterfly, by Parker ('95) in Astacus and by 

 myself in fresh maceration preparations of Cambarus — , possibly a 

 "visual-red"; and since the rays had to pass through this before 



