sie) = The Heliotropism of Hydra. 425 
Total increase 215 to 233,—z.¢., fo per cen 
Single blue (after green), . . rease (per cent) 692 
Double blue (after sr 3 
Triple blue (after yellow) " 436 
Quadruple blue Sen double blue), PR "a 40 
Every light area shows a heavy decrease. The experiment seems to show 
that under the existing conditions the limit of attractiveness, as determined 
by intensity, lies between three and four layers of blue glass. On replacing 
the various blues by red, green, and yellow, as in the table, every colored 
area shows a heavy decrease, and every light area a large increase. 
The general result is that, allowing for all variations of weather, tem 
ture, and irregular movements, Æ. fusca shows a very marked “ preference 
for blue in comparison either with light of other colors or with white light ; 
and an equally marked “ praka ” for white light as compared with any 
color except blue. 
