SPECTRAL ENERGY CURVES OF LIGHT EMITTED BY VARIOUS SPECIES. 19 
The “densities,” i. e., the amount of light absorbed, were than plotted to 
a scale as indicated in Fig. 7. In this illustration the heavy lines (x X, 
x ») give the effect of the light of Photuris pennsyluanica, of Photinus 
pyralis (o- —- --°- —- -©) andof the carbon glow-lamp (-—-—, 4”, 8”, etc.) 
as recorded on the large spectrograph negatives. In the same manner, 
Fig. 8 shows several density curves of Photuris pennsyluanica (@——* *), 
Photinus consanguineus (e——e———®), and of the glow-lamp (-—-— 
------ ), as measured on the negatives obtained with the small spectro- 
graph. In all cases the Photinus density curves extend farther into the red. 
Evidently the time of exposure, i. e., effect on the photographic plate, has 
nothing to do with the question of obtaining an impression in the red. In 
other words, the lack of ‘‘density”’ in the negative of the red end of the 
spectrum is owing to a deficiency in red rays, in the light emitted by the 
Photuris; it is not a lack of intensity of the emitted light. 
T T T T T T T T T 7 T T 
100% Plate 6, 26,11 s ‘ 4 
60-------Zt mor SOS) sons pea: soe 5, 27,711 -,x 
al a ae ae hs eaeshe-== | 6, 18,7 oa 4 
eal a vi ES ce _ Suen as 6, 20,'11 85, 6.6 | 
7 or AS Cn Q lar 
— 
i=} 
T 
Opacity or Density of photographic plate. 
«“ 
nN w 
Q 5 
T T 
3 
T 
It 
SO 1 2 38 4 55 6 7 8 8 60 ' 2 3 4 S&S G6n 
Photuris pennsylvanica { * a 
Photinus pyralis o----9-~~--o---~© 
Glow ‘lamp Sie eS Sees eee co ae ee 
Fic. 7.—Spectrum density curves of photographic plates. 
The two heavy continuous lines are for Photuris pennsylvanice. 
This illustration in itself is an ample demonstration that the light of the 
Photinus pyralis is much richer in red and yellow rays than is the Photuris; 
for the curves intersect, and all those of the Photinus, whatever the density, 
lie to the right of those of the Photuris of equal density. But we can go 
a step farther and eliminate the inequalities of sensitiveness for different 
wave-lengths, by comparing the densities of the negatives. It is assumed 
that the density, or the effect of the light upon the photographic plate, is 
proportional to the time of exposure. This is of course not true for very 
long exposures, and hence the highest parts of the curves are not used in 
this work. For convenience the 4 seconds density curve of the glow-lamp 
was taken as the unit of comparison. Hence the 8 seconds density curve 
represents 2 units, the 12 seconds density curve represents 3 units, etc. 
