METHODS OF INVESTIGATION OF THE LIGHT EMITTED. rE] 
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 1, D, I 14, inwhich the light was entirely of the scintillans. In 
Plate 1, D, I 11, 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, B 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 doesin the red. ‘This 
is better illustrated in Fig. 7, “plate 6:19:’11"’ and ‘‘plate 5:27:’11,”’ which 
gives the photometric densities of Plate 1, B 5 and 6,and 4and 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 1, 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 1,D,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. 
