REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 103 
which the above condition did not hold. Thus Kraus (’76) using 
colored media, discovered that in Claviceps, a fungus, red light 
was nearly as effective as blue, while Engelmann (’82) showed 
by the use of a solar and gas spectrum that Bacterium photo- 
metricum actually collects most readily in the infrared. Further- 
more, the study of various other animals with more refined 
methods began to show that many forms were most affected by 
intermediate points in the spectrum. Thus Yerkes (99) has 
shown that for Simocephalus the point of maximum efficiency 
in a Welsbach gas prismatic spectrum is in the yellow. Bert 
(69) and Lubbock (’81) located this point for Daphnia in the 
green, while recently Loeb and Wasteneys (’16) using the spec- 
trum from a carbon arc, have found the most effective point for 
Balanus larvae in the yellow and that for Chlamydomonas 
pisiformis in the yellow-green. Likewise, Hess, an opthalmolo- 
gist, (10) using the spectrum from a Nernst glower has fixed 
green or yellow-green as the maximum stimulating point for a 
variety of forms, including ichneumon flies, Culex pipiens, adults 
and larvae, Coccinella septenpunctata, Dasychira fascelina and 
cephalopods. In the last instance the reaction of the pupil of 
the eye is taken as a criterion of response. Lastly, 8. O. Mast 
has recently given an excellent summary of work previously 
done and the results of a recent series of experiments of his own 
on Arenicola larvae, blowfly larvae and a number of unicellular 
forms. For the blow-fly larvae the maximum is in the green, 
while for Arenicola it is in the blue. 
From these results it is apparent that the variation in the 
point of maximum response for different animals and plants is 
very wide. ‘To explain this. divergence, the existence in differ- 
ent organisms of different chemical compounds varying respec- 
tively in the degree to which they are altered by light of differ- 
ent wave lengths has been suggested. That there are com- 
pounds of this kind we know, but their presence in phototropic 
organisms has not yet been proved. Aside from this view Hess 
believes that phototropic animals are all color-blind, and that 
they go to the part of the spectrum which seems to them bright- 
est. He apparently gets this idea from the fact that he found 
