1891.] The Helotropism of Hydra. 427 
The curves show very strikingly, along with the indefinite diurnal fluctua- 
tions, the immediate fall in the number of Hydras when placed under any 
color except blue. The curve IV., as compared with that of II., shows 
that the attractive influence of blue, under the conditions of the experi- 
ment, ceased when the intensity of the blue was diminished beyond three 
layers of glass. 
The comparison of curves II., VI., and VIII. shows a remarkable simi- 
larity between them, and indicates that, under the conditions of the experi- 
ments, the actions of red, green, and * yellow”’ did not materially differ. 
EXPLANATION OF TABLE II. (page 428.) 
This experiment gives a comparison of blue, yellow, white, and the 
shadow of an opaque screen (II.), and shows the amount of fluctuation 
from day to day. The general arrangement is the same as in Table I., the 
same aquarium, Hydras, position, and areas being used as before, but the 
areas are increased in number, so as to extend over nearly the whole illu- 
minated side, area I. being three mm. from one end, and area XVI. nine 
mm. from the other. The comparison is made between the first and last 
observations 
Total increase, = to oe é., 60 op cent. 
Blue, increas . à a 2387 per cent. 
Yellow, Pien crys 30 
Dark screen, decreas 37 
Light (a mean of V., Vi. VIL, increase L et: 
An inspection of the table shows that although these figures express the 
broad general result with sufficient accuracy, they are not to be taken 
to mean more than this, since there is a wide margin of apparently 
fortuitous variation from day to day. The table shows a marked “pre refer- 
ence ” for the blue, and a much less marked but still distinct “ preference 
for ordinary daylight, as compared with the light of diminished intensity 
ka the opaque screen. The yellow glass acts practically as if it were 
opa 
