18 A PHYSICAL STUDY OF THE FIREFLY. 
VII. SPECTRAL ENERGY CURVES OF THE LIGHT EMITTED BY 
VARIOUS SPECIES OF FIREFLIES. 
The photographs just discussed show that some fireflies emit light which 
is much richer in the red rays than obtains in the light of other species. It 
is important to know whether this is due simply to adifference in the shape 
of the spectral energy curve, or whether the maximum emission is different 
in the various species. ‘To obtain this information it is necessary to elimi- 
nate the unequal sensitiveness of the plate for different wave-lengths, and 
also the change in the density exposure relationship for different colors. 
This is accomplished by finding the “densities” or ‘‘blackening’’ of the 
photographic plate, for which purpose a Martens polarization photometer 
wasemployed. ‘Che photometer was substituted for one of the microscopes 
on a small comparator, as shown diagrammatically in Fig. 6. A beam of 
light from the standard lamp, J, passed directly into the photometer, Q. 
A second beam of light, on reflection from the mirror, M, passed through 
plate, N, to be examined, through the slit, R, and into the photometer. 
i 
Fig. 6.—Side view of] photometer used in measuring densities of the photographic plates. 
Although this was not absolutely necessary, a standard plate, P, was kept 
on the comparator, for reference in case of accident to the adjustments. 
The slit, R, was placed as close as possible (about 0.2 mm.) to the plate, 1, 
so as to take in the width desired and no more. ‘The width of the slit, R, 
was 0.5 mm. for the small spectrometer and 1 mm. for the large instrument. 
Lengthwise on the negative, NV, was placed a strip of black paper with a slit 
in it, which was somewhat longer than the photograph of the spectrum. 
This slit was 1.5 mm. wide for the large spectrometer, and 1 mm. wide for 
the smaller instrument. ‘he microscope and scale, S, were used to make 
settings on the different parts of the negative. In thismanner, for the nega- 
atives obtained with the large spectrograph, strips 1.5 mm. high and 1 mm. 
long were compared against the clear unexposed plate. In the same manner 
for the negatives obtained with the small spectrograph, strips 1 to 1.2 mm. 
high and 0.5 mm. wide were photometered. One of the helium lines, illus- 
trated in Plate 1, B, 1 was used as a reference standard and the points of 
measurement were the same on all the plates. 
