358 



THE CANADIAN ENTOMOLOGIST. 



curves have the same density, which in magnitude, by our arbitrary scale, 

 is I unit. At .532/x and at .572/^ this same Photuris curve intersects the 

 8 seconds density curve of the glow lamp, and hence, on our arbitrary 

 scale, photometric value at these two points is 2 units. At the intersec- 

 tions with the 20 seconds density curve of the glow lamp the photometric 

 value is 5 units, etc. These photometric values, r, 2, 5, etc., or ratios of 

 densities, are plotted to scale in fig. 25. The dotted curve in this illustra- 

 tion is plotted from Langley's photometric values 01 Pyropko?'us no-ctilucus. 



Fig. 25. — Ratio oF densities of photographic plates 

 fire-My light 



glow lamp light 



The fire-fly curves have now been compared against a standard source 

 and their maxima are entirely different It is of interest to note that the 

 glow worm (larva of the Phoiuris pennsyhajiica) has its maximum at 

 practically the same place as has the adult insect. In these curves the 

 various circles (o, 6, o), etc., indicate that the resultant curve is the com- 

 posite of numerous "density" curves. (See fig. 23.) The photographic 



