624 
ZOOLOGY: S. O. MAST 
efficiency of the different colors can be ascertained. To do this all that 
is necessary is to vary the luminous intensity of the white light for each 
change in the colored light until, in each case, the organisms proceed on 
the same path. The stimulating effect of the different colors will then 
be directly proportional to the various luminous intensities of the white 
light required to make the organisms under each of the different condi- 
tions, proceed in the same direction, e.g., if for green it required twice 
as much light from the white source to make the creatures take a given 
course as it does for yellow, then the stimulating effect of the green is 
twice as great as that of the yellow. To ascertain the relative efficiency 
in terms of wave-lengths and energy it is only necessary to use a spec- 
trum having a known distribution of energy, and to make corrections 
in accord with this distribution. 
In the experiments referred to below, two gas-filled street-series 
tungsten lamps with coiled filaments were used in series to produce 
two beams of light. One of these beams passed through a Hilger con- 
stant deviation spectrometer and the other through a Lummer-Brodhun 
rotating sector. The whole apparatus was so arranged that the two 
beams of light crossed at right angles in the field of observation. For 
every color tested the intensity of the illumination in the beam of white 
light was adjusted by varying the opening in the sector, until the course 
of the organisms bisected the angle between the two beams of light. 
In nearly all cases the successive regions selected in the spectrum 
differed by 10 mm. In this way the relative stimulating effect for the 
different regions of the spectrum was ascertained in fifteen different spe- 
cies as follows: Chlamydomonas, Trachelomonas and Phacus, each one 
species; Euglena, five species; Panderina, Eudorina, Gonium and Spondy- 
lomorum, each one species; earthworms, Arenicola (larvae) and blowfly 
(larvae) each one species. All but the last three are green, microscopic 
organisms, relatively very simple in structure. 
The results obtained will be stated in terms of relative stimulative 
effects of the different regions of the spectrum tested without correc- 
tions for the difference in the energy of these regions. They are, how- 
ever, of such a nature that the corrections mentioned will not result 
in marked alterations. These corrections will appear in the final paper. 
For all but one of the microscopic organisms the results fall into two 
groups. In the one group the region of stimulation begins in the blue 
near the violet, between 430 and 440 At/x. From here toward the red end 
of the spectrum the stimulating efficiency rises, at first slowly and then 
rapidly, to a maximum in the green near the yellow, between 530 and 
540 /x/x; then it falls, at first rapidly and later more and more slowly, 
