16 A PHYSICAL STUDY OF THE FIREFLY. 
1 meter. When used with a light flint-glass prism the dispersion in the 
yellow was sufficient to separate the yellow mercury by 0.5 mm., the total 
length of the spectrum between the green (\=o0.5014) and the red (A= 0.6671) 
helium lines being 37 mm. ‘The firefly light falls within this same region. 
A small spectrograph, of triple achromatic lenses 6 cm. diameter and 18 
em. focal length, which had a much greater light-gathering power, was 
used in photographing the weak radiations in the red. ‘The same prism (of 
light flint-glass) was used on both instruments. In the large spectrograph 
the slit opening was 0.75 mm. and in the small instrument it was 0.4 mm. 
The collimator slit was covered with a small piece of white cardboard con- 
taining an opening 5 mm. high and 2 mm. wide, which enabled the operator 
to center the fireflies over the slit, by means of their own light; for, asalready 
mentioned, the photographing was done in a completely darkened room. 
‘The method of procedure consisted in holding the insects in the fingers, 
one or two at a time, over the spectrographic slit. The constant struggle of 
the insect to get away caused it to flash more frequently than is its accus- 
tomed rate. If it became quiet and ceased to flash, a movement of the 
fingers, or allowing the insect to move about, increased the frequency of the 
flashing. Frequent flashing is an important item when it comes to making 
exposures to weak sources like the firefly, requiring from 1 to 5 hours to 
obtain a satisfactory negative. 
In the small spectrograph only from 1 to 60 minutes were required to 
obtain a uniformly exposed, dense negative. On the small apparatus a 
single flash from the Photinus pyralis was sufficient to cause a streak across 
the plate (see Plate 1, D, V 12; and VII 5, 6, and 8), and in order to obtain a 
uniformly exposed negative, the slit was covered with a piece of ground 
glass. In Plate 1, D, V 2 the negative was obtained in 2 minutes from 50 
flashes of pyralis through ground glass; III 2 was similarly obtained from 8 
flashes; III 5 from 20 flashes; III 6 from 35 flashes; III 7 from 60 flashes; 
III 8 from 5 flashes; and III 9 from too flashes or about 4 minutes. On the 
other hand, the negative directly below this one, III 10, required 30 minutes 
exposure (without glass) to a male Photuris pennsylvanica which flashed at 
the rate of about 100 times per minute. 
As indicated elsewhere, the longer time required for exposure to the 
Photuris is not because of the lack of intensity, but because of the shorter 
(as compared with pyralis) duration of the flash. Although the luminous 
organs in the two species are different, those of the Photuris being the smaller, 
the area covered on the spectrograph slit is not markedly different in the 
two insects. 
On the spectrometer slit, the Photinus scintillans and consanguineus could 
be made to flash about 4 to 5 times per minute. The Photinus pyralis 
flashed about 20 times per minute when at its best, but the flash lasts longer 
and therefore (aside from its greater volume) has a greater effect on the plate 
than the flash of the Photuris pennsylvanica. Inthe Photuris pennsylvanica 
the flash, accompanied by a “twinkling effect,’ often occurs at the rate of 2 
to 3 times per second, which in some male specimens was sometimes con- 
tinued for an hour or more with but few interruptions. 
Only a few specimens of Photuris glowed vigorously during the photo- 
graphing. The Photinus pyralis easily tires of flashing and then emits a 
