430 The American Naturalist. [May, 
difficult to fix exactly the limit of the attractive rays. As nearly 
as can be determined they extend over the lower third of the 
blue end,—i. e., from G nearly to F—and for a short distance 
into the green. 
The results of these experiments leave no doubt that, irrespec- 
tive of intensity; Hydra prefers’ blue light to all other colors 
and to white light (ordinary daylight). My observations indicate 
further that, although the blue rays are by far the most efficient, 
a slight attractive influence is also exercised by the green. Under 
ordinary circumstances—/. ¢., when diffused daylight is not cut off 
from behind or above—Hydra appears to be as indifferent to green 
as to red or to an opaque screen. If, however, the animal be 
enclosed in an aquarium so arranged as to offer it the choice 
between green and either red or “ yellow,” a distinct though slight 
preference is shown for the green, and the animals very gradually 
accumulate behind it. The green glass used in this experiment 
shows no trace of blue under the spectroscope. If the choice be 
offered between red or “ yellow” (the latter = red + yellow + 
green), no perceptible preference is shown, even if the experiment 
be continued for weeks. This result is of some interest, for It 
seems to show that the slight attractive influence of green © 
nullified by the admixture of red and yellow, just as the attrac- 
tiveness of blue is diminished by the admixture of the other — 
colors, as has been shown. ‘ 2 
The preference of Hydra for blue as compared with white 
light is a very remarkable fact; for the animal can never have 
had any experience of pure blue, but only of white, light, t t» 
blue plus the other colors of the spectrum. Neither can ti 
preference for blue glass be due simply and solely to the attractive = 
influence of the blue rays, for the ordinary daylight ente = 
the aquarium contains at least as many blue rays 8S her 
valaenes Arcee {about two feet from the prism) was about the inches ee 
meats nated n e onto ark terest a 
hee 
hy th. g 
iat i 
that proceeding from the prism. Eo 
?The word “ prefer” is perhaps objectionable as implying an act of gee 
the part of Hydra. I do not wish to make such an implication, h ' ae 
word only for the sake of brevity. 
