422 The American Naturalist. [May, 
The same result is reached if two or even three layers of blue 
glass are used (see Table II.), although in the last case the blue 
color is so dark that at a short distance it appears nearly opaque 
to the eye. It.is, moreover, immaterial whether the four (three) 
areas constitute the only source of light (the top and other sides 
being in this case covered with black paper) or the diffused light 
of the room enter from behind and above; the result remains the 
same. Red and green glass agree nearly with yellow, the Hydras 
treating them practically as if they were opaque. (This statement 
will require some modification hereafter.) 
The result thus obtained is rendered still more striking if the 
yellow and blue glasses be interchanged. Within an hour or two 
the Hydras begin to move out of the yellow light and into the 
blue, and in a day or two, more or less according to circumstances, 
the numbers under the blue are far in excess. Thus the Hydras 
may be driven from one area to another and back again by inter- 
changing the glasses, as often as may be desired (see Table HI). — 
For further details the reader is referred to the explanation of 
the tables and the ‘chart. 
BF, E ae Mh wo ott gee EA Siw SAES NNA AL 
