SPECTRAL ENERGY CURVES OF LIGHT EMITTED BY VARIOUS SPECIES. 21 
two samples were obtained from the photographs taken with the small 
spectrograph, Fig. 8. Knowing the distribution of energy in the spectrum 
of the glow-lamp given in Fig. 10, it is possible to determine the spectral 
energy distribution of the firefly by multiplying the energy values of the 
low-lamp by the ratio of densities = aeny EBA from Fig. 9, at each 
8 Pe ’ glow-lamp light ’ oe 
wave-length. The resultant curves are given in Figs. 10 and 11 and tabu- 
lated in Tables 1 and 2. 
In Fig. 10 the spectral energy curve of Photuris pennsylvanica and Pho- 
tinus pyralis are plotted to the same scale in the blue-green. 
An integration 
- T T T T 
* Plate 5,26,'Il 
S279 Ih = 
Tr 
6,19, lod 
620,711 & 
6 2 
ia, fo) 
>| B Ey 
aie ce 
O lew rm 
= 
HES ; 
1) ® 
=) 
i e. 
ds : 
E i 
‘a 
Ae 3 
£2] 
fo) 
° 
= 
< 
G 2 4 
| 
rl 1 
52 4 6 8 60 2 abe 
Fic. 9.—Ratio of densities of photographic plates (firefly light+glow-lamp light) 
obtained from the preceding illustrations. 
of these two curves shows that for the same emissivity in the blue the energy 
curve of the Photinus pyralis is 2.83 times that of the pennsylvanica. To 
the eye it is apparent that the illuminating power of the Photinus is far 
greater than that of the Photuris. Whether the Photuris pennsylvanica 
curve is asymmetrical, as indicated in Fig. 10, or symmetrical is difficult to 
decide because of the far greater sensitivity of the plate for the region of 
0.59u, and the fewness of the negatives. The same irregularity occurs in 
the Photinus pyralis, but is somewhat obliterated by the nearness of the 
maximum of emission. 
