1891.] The Heliotropism of Hydra. 431 
light after its passage through the blue glass. The conclusion- 
would seem to be inevitable that the lower rays exercise an 
injurious or repellant action, and thus tend to produce negative 
heliotropism, or to counteract the effect of the blue rays. Itis a 
tempting hypothesis to suppose that the blue rays are most 
efficient in light of low intensity, and the lower rays most efficient 
in high,—a view which would explain in the clearest manner the 
reversal of heliotropism with the change in intensity. Experiment, 
however, does not sustain this conclusion, but indicates that the 
animal is wholly indifferent to the lower rays. Hydras supplied 
only with light that has passed through red or yellow glass do 
not noticeably move either away from or towards it, but behave 
as though the glass were opaque. Tested with the actual 
spectrum, they appear to be quite indifferent to all of the rays 
except the lower blue and the upper green. I have also tested 
this question by the comparison of nearly pure blue glass with 
purple (aqueous solutions of methyl-violet of various intensities), 
which is a mixture of blue and red. Any repellant action on 
the part of the red might reasonably be expected to counteract 
more or less completely the attractiveness of the blue. Experi- 
ment shows, however, that purple is as attractive as pure blue, 
—neither more nor less, as far as can be determined. 
It appears, therefore, to sum up, that although the lower rays 
are without any perceptible action on Hydra, by themselves or 
when mixed singly with the upper rays (as in purple), yet they 
partially counteract the attractiveness of the blue rays when 
mixed with them as they are in ordinary daylight, and of the 
green rays when mixed with them so as to form yellow (é. e., white 
light minus blue). This paradoxical result I am at present 
unable to understand, but the problem is undoubtedly worthy of 
the most careful investigation. 
Why the blue-green rays alone should be operative it is 
impossible to say. The recent works of Loeb and Groome upon 
animal heliotropism, -and the earlier work of Sachs, de Bary 
and others upon plants, show that in all probability the blue rays 
are the effective ones in all cases of heliotropic action, whatever its 
Purpose or mode of origin, whether in plants or animals, whether 
