METHODS OF INVESTIGATION OF THE LIGHT EMITTED. 15 
The photographic plates were imported fresh from the factory about a 
month before work was begun. It is, of course, impossible to obtain all the 
glow-lamp and firefly exposures upon one plate, and since they were all from 
the same batch as sensitized in the factory it was assumed that all the plates 
have the same “characteristic curves” as those obtained from the spectrum 
of the carbon lamp photographed upon one plate. That this assumption 
was permissible is shown by the excellent agreement of the spectral energy 
curves of the light of the firefly as obtained from different plates taken from 
the same box and from different boxes. 
More important than the difference in the plates is the question of devel- 
opment. All plates were developed, under identically the same conditions, 
by time, in complete darkness, the developer and fixing bath being kept cool 
to prevent the softening of the gelatin. The negatives should therefore be 
comparable without all the exposures being made on one plate. In fact, 
when using the large spectrograph, there was usually a long and a short 
exposure of a given species of firefly, from which it was possible to obtain the 
complete energy curve from one plate. Experience showed that, although 
the photographing was done in a well-darkened room, when the plate was 
in the holder for from 4 to 6 hours it became “‘fogged,’’ and hence, when 
photographs were taken during the daytime, no attempt was made to obtain 
more than two negatives on one plate. Such plates, when developed, were 
entirely free from “‘fog.’’ No risks were taken in developing the plates, 
fresh developer (metol-guinol) being made up each time. The agreement of 
the photometric curves of the light of the same insect, obtained from differ- 
ent plates, indicates that but little would have been gained by attempting 
to photograph different species on the same plate. 
In the vicinity of the Bureau of Standards, District of Columbia, the 
flight of the Photuris occurred the first week in June, and no Photinus pyralis 
could be found until the last week in June, when but few examples of Pho- 
turis were to be seen. However, by bringing Photinus pyralis from some 
distance* (in a different part of the city of Washington some 3 miles away, 
where the pyralis appeared in the middle of June), it was possible to obtain 
the short-time exposures of various species on the same plate (Plate 1, D), 
using the small spectrograph to be described presently. 
The proper timing of the exposures was found impossible, other than the 
rough estimate of exposing the plate to the pyralis from 1 to 2 hours, and 
about four times as long to obtain the same impression from the Photuris. 
An excellent example of the uncertainty in the matter is shown in Plate 1, 
B, spectra 4and 7. The former was obtained in an hour and the latter in 
3% hours. It was intended to have the two negatives widely different in 
densities, but they are identical in density (see Fig. 7, ‘‘plate 5:27:'11’’). 
On one occasion a 53-hour exposure did not give as dense a negative as the 
one recorded in Fig. 7, ‘plate 5:26:11." This is owing to the fact that after 
being in captivity for a few days the activity of the insect is decreased and its 
flash does not seem to be so intense. 
‘Two spectrographs were used in obtaining the photographs of the firefly 
light. The one was a large quartz-lens instrument having a focal length of 
*Especial acknowledgment is due to Mr. U. F. Rosen for his faithfulness in catching 
these specimens, and for valuable aid throughout this season’s work. 
