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 0.520JU and at 0.592/1; i. e., at these two points the two curves 

 have the same density, which in magnitude on our arbitrary scale is i unit. 

 At 0.532/i and at 0.572/i 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 so on. These photometric values, i, 2, 5, etc., or "ratio of densities, " 

 are plotted to scale as ordinates in Fig. 9, and the corresponding wave- 

 lengths as abscissae. The dotted curve in this illustration is plotted from 

 Langley's photometric values of Pyrophorus noctilucus. The firefly curves 



4 6 8 .60 2 4 6 



Photuris pennsylvanica • •- 



Photinus consanguineus © hs)- 



Glow lamp 



JQ/LL 



Fig. 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 (®, ©, ^' 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- 

 able factor. The photographic plate is very sensitive in the region of 0.590/i 

 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/i is therefore not considered of real significance. The curves of these 



