14 A PHYSICAL STUDY OF THE FIREFLY. 
adult state. It would also be interesting to examine sections of Photuris 
after the insect has flashed for an hour and a half or longer, as described 
elsewhere, and of a similar specimen which has not undergone such a period 
of flashing, to see whether the ‘reflecting layer”’ is markedly different in 
the two samples. Of course numerous examples of each (of the same age) 
would have to be examined to obtain an average value. 
VI. METHODS OF INVESTIGATION OF THE LIGHT EMITTED. 
As with other sources of radiation, the best method of investigating 
the composition of the light of the firefly appeared to be one involving the 
determination of the spectral energy curve. Because of the weakness and 
intermittence of the radiation it is not possible to use a bolometer or a spec- 
trophotometer, and the only satisfactory method is the one involving pho- 
tography. While this seems to be an indirect method to obtain a spectral 
energy curve, it will be noticed presently that it is not so cumbersome as it 
appears. The photographic plate, being integrative in its action, is well 
adapted to the investigation of weak and intermittent sources, but it must 
be of a special kind which is sensitive to all parts of the visible spectrum. 
The nearest approach to this condition is the Wratten and Wainwright 
“Panchromatic’’ plate, which is sensitive, in a variable degree, to all the 
frequencies from the ultra-violet far into the red. The method is really a 
species of spectrophotographic photometry in which the light of the firefly 
and that of the standard source are photographed, after which the “densi- 
ties’’ of the negatives are compared. 
In ordinary spectrophotometry one compares intensities. In the photo- 
graphic method the effects of these intensities on the photographic plate are 
compared, the element of time being the unit of measure and the effect of a 
given intensity upon the photographic plate being taken proportional to 
the time of exposure. This, of course, is not true for long exposures. For 
a limited range the density of the plate is proportional to the time of expos- 
ure; beyond this range saturation begins and, still farther away, reversal 
takes place. It is therefore necessary to determine the exposure equivalent 
of each density measured; that is to say, the ‘‘characteristic curve’’ of the 
plate must be determined. This is accomplished by making a series of 
exposures of known relationship of a standard source in which the densities 
cover the same range as those obtained on the unknown source, e. g., the fire- 
flies. Such a series of exposures are shown in Plate 1, A, the standard 
source being acarbon glow-lamp operated on 4 watts per candle. Thelamp 
was placed at a fixed distance from the spectrograph slit, which was covered 
with a ground-glass plate, the voltage was kept constant, and a series of 
exposures was made corresponding to 2, 4, 6, 8, 10, 12, 20, 30, 60, 120, and 
240 seconds. For densities on the firefly negatives, in which there were no 
corresponding density measurements on the glow-lamp, the characteristic 
curve of the latter was drawn for the particular wave-length in question, 
and the firefly density value was determined by interpolation. 
Since the energy value, ‘‘mechanical equivalent,” is not the same for all 
rays, it is necessary to know the spectral energy distribution of the carbon 
glow-lamp. This was determined anew for the present research by means of 
a large vacuum spectrobolometer, fluorite prism, and mirror spectrometer. 
