104 ROBERT STANLEY McEWEN 
so many organisms for which yellow-green is the most effective 
part of the spectrum, this being also the brightest part for color- 
blind men. This notion, has been criticized. by Ewald (’15) 
and Loeb (16). 
The peculiar fact about Drosophila is the reversal in the ef- 
fectiveness of red and green as the insects’ eye color grows darker. 
Thus for eye colors lighter than normal the order of effectiveness 
is violet, green, red, while probably for normal, and certainly for 
sepia, the order is violet, red, green. This case besides showing 
the peculiar reversal is remarkable as being the only instance so 
far discovered among the lower animals in which red is more 
effective than green, with the possible exceptions of Daphnia 
(Frisch and Kupelwieser, 713; Ewald, ’14), and paramoecium 
bursaria (Engelmann, ’82). How to account for this phenomenon 
of reversal it is difficult to say. Were it not for the case of sepia 
it might be explained on the basis of the changed amount of red 
reflected by the normal colored rhabdomes as compared with that 
reflected by those of lighter shades. When two shades produce 
the same effect, however, it is difficult to see how this will suffice. 
It would thus seem as though we must fall back on the assump- 
tion that as the eye grows darker, the supposed sensitive chemi- 
cal substances on which the light has its effect change also. 
_ What this change could be, it is hard to imagine from what we 
now know of photo-chemical reactions. I am inclined to think, 
therefore that the explanation may yet be found in connection 
with some sort of differential absorption. 
It may be noted that my results with colored lights do not 
agree in one respect with those of Dr. A. O. Gross who also 
worked on Drosophila This writer found green more effective 
than red for flies with normal eyes, while my experiments re- 
versed this order. I suggest, however, that this descrepancy is 
due to the fact that Dr. Gross used lights which were equated 
in energy, whereas in the case of my filters, as is also true for the 
normal spectrum, the energy of the red is much greater than 
that of the green. This fact, nevertheless, does not invalidate 
or make less interesting the very evident increase in the effective- 
ness of red in the case of the darker eye colors, since whatever 
