METHODS OP INVESTIGATION OF THE UGHT EMITTED. 17 



strong glow which usually is the signal of the end of its usefulness. If, 

 however, it is placed in an inclosure and allowed to rest, it often resumes the 

 flashing. As stated elsewhere, the flashing is under the control of the insect, 

 unless injured or overexerted when the luminous organs begin to glow. 

 This glow may be in only one segment. Sometimes the glow extends over 

 one whole segment and part of the other segment. The non-glowing part 

 of the latter will then be found to emit its flash as usual ; and in one case it 

 was found that this local flashing continued for about 10 minutes. The 

 application of a slight pressure caused a continuous glow to be emitted also 

 from this part. 



The Photinus scintillans is so small that it is handled with great difficulty. 

 Its flashes are few, but it emits a strong glow. Only one photograph was 

 made, Plate i, Z>, I 14, in which the light was entirely of the scintillans. In 

 Plate I , D, I II, a few glowing scintillans were used, the negative being 

 finished with consanguineus. 



In order to make certain that at least some of the negatives would prove 

 suitable for demonstrating the great difference in the light emissivity, the 

 method adopted was to take one or two exposures of the Photuris and to fill 

 in the rest of the plate with the light of the Photinus pyralis, which required 

 less time to obtain an exposure. In this manner it was possible to obtain 

 negatives of the pyralis which were both more and less dense than the photo- 

 graphs of the Photuris. From these photographs it was easy to see that the 

 light of the Photuris does not extend so far into the red as does the Photinus. 

 This is well illustrated in Plate 1,^4 and 7 (Photuris) and 5 and 6 {Photinus). 

 If it were simply a question of "density " or time of exposure, then 5 should 

 have extended farther into the blue than 4, just as it does in the red. This 

 is better illustrated in Fig. 7, "plate 6:19:'! i" and "plate 5:27:' 11," which 

 gives the photometric densities of Plate 1,^5 and 6, and 4 and 7, respectively. 

 Here it is shown that, for the same density in the blue, the Photinus shows a 

 far greater abundance of radiation in the red than obtains in the Photuris. 

 In Plate i, D, the scintillans, I 14, and the pennsylvanica, I 13, are of the 

 same density (by actual measurement) in the yellow, yet the former lies the 

 farther toward the red. Similarly, Plate i,Z),the pyralis, II 7 and 8, penn- 

 sylvanica, II 9 and 10, and consanguineus, II 11, have closely the same den- 

 sity in the yellow ; and a mere visual inspection shows that the maximum of 

 the radiation of the pennsylvanica lies farther toward the blue than obtains 

 in the other species. All these negatives show that the less dense Photuris 

 extends farther toward the violet than does a more dense Photinus, which 

 would not be possible if the light of the two species had the same composition 

 and differed merely in intensity. So much emphasis is placed upon this 

 point for the reason that, among those with whom the writer has discussed 

 the matter, the predominating opinion is that the whole effect upon the eye 

 is, and should be, the result of a difference in intensity. 



