i8 



A PHYSICAL STUDY OF THE FIREFLY. 



VII. SPECTRAL ENERGY CURVES OF THE LIGHT EMITTED BY 

 VARIOUS SPECIES OF FIREFLIES. 



The photographs just discussed show that some fireflies emit light which 

 is much richer in the red rays than obtains in the Hght of other species. It 

 is important to know whether this is due simply to a difference in the shape 

 of the spectral energy curve, or whether the maximum emission is different 

 in the various species. To obtain this information it is necessary to elimi- 

 nate the unequal sensitiveness of the plate for different wave-lengths, and 

 also the change in the density exposure relationship for different colors. 

 This is accomplished by finding the "densities" or "blackening" of the 

 photographic plate, for which purpose a Martens polarization photometer 

 was employed. The photometer was substituted for one of the microscopes 

 on a small comparator, as shown diagrammatically in Fig. 6. A beam of 

 light from the standard lamp, /, passed directly into the photometer, Q. 

 A second beam of light, on reflection from the mirror, M, passed through 

 plate, N, to be examined, through the slit, R, and into the photometer. 



Fig. 6. — Side view of photometer used in measuring densities of the photographic plates. 



Although this was not absolutely necessary, a standard plate, P, was kept 

 on the comparator, for reference in case of accident to the adjustments. 

 The slit, R, was placed as close as possible (about 0.2 mm.) to the plate, N, 

 so as to take in the width desired and no more. The width of the slit, R, 

 was 0.5 mm. for the small spectrometer and i mm. for the large instrument. 

 Lengthwise on the negative, iV, was placed a strip of black paper with a slit 

 in it, which was somewhat longer than the photograph of the spectrum. 

 This slit was 1.5 mm. wide for the large spectrometer, and i mm. wide for 

 the smaller instrument. The microscope and scale, S, were used to make 

 settings on the dift'erent parts of the negative. In this manner, for the nega- 

 atives obtained with the large spectrograph, strips 1.5 mm. high and i mm. 

 long were compared against the clear unexposed plate. In the same manner, 

 for the negatives obtained with the small spectrograph, strips i to 1.2 mm. 

 high and 0.5 mm. wide were photometered. One of the helium lines, illus- 

 trated in Plate i, B, i was used as a reference standard and the points of 

 measurement were the same on all the plates. 



