THE CANADIAN ENTOMOLOGIST. 



857 



fig. 24, where the horizontal scale is the wave length. In this illustration 



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.66. 



Fig. 24. — Density curves of photographic plates of 

 Photuris peniisylvajiica X X X 



Photmiis py rails ^ 

 Glow Lamp — 



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the heavy black lines give the effect of the light of PJioturls peJinsylvaiilca 

 (X X X, ., c/> (/) (/)), of Photltius 



py rails {0- 



■0- 



0), and of the carbon glow lamp (- 



This illustration, in itself, is an ample demonstration that the light of 

 the Photmus pyralls is much richer in red and yellow rays than is the 

 Photuris ; for the curves intersect, and all those of the Photlnus, whatever 

 the density, lie to the right of those of the Photuris of equal density. But 

 we can go a step farther by comparing 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 I have taken the four seconds glow 

 lamp density curve as my unit. Hence the 8 seconds density curve 

 represents 2 units, the 12 seconds density curve represents 3 units, etc. 

 Turning now to the fire-fly curves we see that the upper, heavy, density 

 curve of Photuris pennsylvanica intersects the 4 seconds density curve of 

 the glow lamp at .52/x and at .592/x; i.e., at these two points, the two 



