DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 321 
As regards blue-violet, green, and yellow, what evidcnce there is 
points to the fact that they stand close together, but of the three 
blue-violet is possibly the more efficient in eliciting the migration of 
pigment. 
Having discovered a d-cided difference in the efficiency of blue- 
violet and red, it was suggested that some quantitative expressioi 
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 seccnd method 
employed in the experiments, viz. by direct observation of the changes 
in the glow iuduced 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 réle 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 proximal! 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 
