ZOOLOGY: S. O. MAST 
625 
ending in the red at about 640 mm. In the other group the region of 
stimulation begins in the violet between 420 and 430 fjLfi, only a short 
distance from the place where it begins in the first group. From here 
the efficiency rises very rapidly reaching a maximum in the blue between 
480 and 490 aim- It then falls rapidly and ends in the green in the 
neighborhood of 520 mm. Three of the microscopic forms, Pandorina, 
Eudorina and Spondylomorum, belong to the first group, the rest to the 
second. To this group belong also Arenicola larvae and the earthworms. 
For the remaining microscopic form (Chlamydomonas) the maximum 
is in the green very near 510 mm; and for the blowfly larvae it is approxi- 
mately at 520 MM- The distribution in the spectrum, of stimulating 
efficiency is, for this creature, essentially the same as the distribution of 
brightness for totally color-blind persons. No difference in the relative 
effect of the different wave-lengths was discovered in any given species 
under different conditions. It was the same for organisms collected in 
different regions at different periods of the day and tested under various 
conditions of illumination and temperature, and it was the same for 
negative and positive individuals. 
These results show that stimulation in all of the organisms studied 
depends upon the wave-length of the light; that the stimulating efficiency 
is very much higher in certain regions of the spectrum than in others; 
but that the distribution of this in the spectrum differs greatly in certain 
organisms that are closely related in structure, e.g., Pandorina and 
Gonium, while it is essentially the same in others that are very different 
in structure, e.g., Euglena and earthworms. They show, moreover, 
that if the absorption throughout the spectrum is the same in the differ- 
ent organisms the chemical or physical changes associated with the 
reactions differ in some species which are closely related. And that 
the changes in the organism, whatever they may be, which cause changes 
in the sense of the reactions are not reversible; for if they were reversible 
one would expect the distribution of stimulating efficiency in the spec- 
trum to differ in positive and negative specimens of the same species. 
[This article is a contribution from the Nela Research Laboratory, 
National Lamp Works of the General Electric Company, Nela Park, 
Cleveland, Ohio.] 
