20 A PHYSICAL STUDY OF THE FIREFLY. 
Turning now to the firefly curves we see that the upper, heavy density 
curve of Photuris pennsylvanica intersects the 4 seconds density curve of the 
glow-lamp at 0.520 and at 0.592y; i. e., at these two points the two curves 
have the same density, which in magnitude on our arbitrary scale is 1 unit. 
At 0.532u and at 0.5724 this same Photuris curve intersects the 8 seconds 
density curve of the glow-lamp and hence, on our arbitrary scale, its photo- 
metric value at these two points is two units. At the intersections with the 
20 seconds density curve of the glow-lamp, the photometric value is 5 units; 
and soon. ‘These photometric values, 1, 2, 5, etc., or ‘‘ratio of densities,”’ 
are plotted to scale as ordinates in Fig. 9, and the corresponding wave- 
lengths as abscissee. ‘he dotted curve in this illustration is plotted from 
Langley’s photometric values of Pyrophorus noctilucus. ‘The firefly curves 
100% 
wo 
oO 
@ 
oO 
~ 
o 
T 
aD 
oO 
a 
fo} 
aN 
° 
Opacity or “Density” of photographic plate. 
i n 1 1 ! l i 1 1 
SO 2 4 6 8 60 2 #* 6 8 70M 
Photuris pennsylvanica »——~»e——. 
Photinus consanguineus e—~o——e 
Glow lamp -------+------ 
Fic. 8.—Spectrum density curves of photographic plates. 
have now been compared against a standard source, and their maxima are 
found to be entirely different. It is of interest to note that the glow-worm 
(larva of the Photuris pennsylvanica) has its maximum at practically the 
same place as has the adult insect. 
In these curves, Fig. 9, the various circles (0, 4, 6 etc.) indicate that 
the resultant curve is the composite of numerous “ratio of densities” curves 
(see Fig. 7), obtained by multiplying each “‘ratio of density” curve by a suit- 
ablefactor. ‘The photographic plate is very sensitive in the region of 0.590 
and it is difficult to eliminate this effect in the firefly curves. The hump in 
the curve of the light from the glow-worm and of Photinus consanguineus at 
0.59 is therefore not considered of real significance. ‘I'he curves of these 
