THE CANADIAN ENTOMOLOGIST. 355 



THE COLOUR OF THE LIGHT EVHTTED BY LAMPYRID.^:. 



BY W. W. COBLENTZ, WASHINGTON, D. C. 



The light emitted by various animals has been described as being" 

 *'blue," "bluish," "green," "yellowish," "orange-coloured," "red," etc. In 

 view of the doubts'*^ as to whether this colour is a subjective phenomenon, 

 resulting from the variation in colour sensibility of the eye with variation 

 in intensity, or whether it is an objective reality, it seemed of interest to 

 attempt to settle the question by subjecting the light to physical analysis 

 by means of instruments which are unbiassed in the matter. One can, of 

 course, examine the light visually by means of a spectroscope and note 

 that in some cases it is "bluer," and on other cases it "extends further 

 toward the red," etc., but to those familiar with the vagaries of the human 

 eye, such a record is useless. The proper instrument to use is a bolo- 

 meter, but because of the lack of sensitiveness of such an instrument and 

 because of the intermittence of the light, the spectral analysis of the light 

 by such means is prohibited. Spectrophotometric comparisons are also 

 precluded. The only satisfactory means available is the photographic 

 plate, which is integrative in its action, and hence well adapted to study 

 weak sources of radiation. The photographic plate must be of a special 

 kind, sensitive to all parts of the visible spectrum. The nearest approach 

 to this condition is the ^Vratten and Wainwright "Panchromatic" plate, 

 which is sensitive, in a variable degree, to all frequencies from the ultra- 

 violet far into the red. The method is really a species of spectro-photo- 

 graphic photometry, in which the light of the fire-fly and that of a standard 

 source are photographed, after which the "densities" of the negatives are 

 compared, in a manner to be described presently. 



The apparatus used consisted of a large prism spectrograph of one 

 meter focal length, giving a long spectrum, and a small spectrograph, of 

 triple achromatic lenses, 6 cm. diameter and i8 cm. focal length, which 

 had a much greater light-gathering power, hence useful in photographing 

 the weak radiations in the red. The method of procedure consisted in 

 holding the insects in the fingers, one or two at a time, over the spectro- 

 meter slit. This required an exposure varying from one to five hours on 

 the large spectrograph, and from one to sixty minutes on the small 



*Knab. — Canadian Entomologist, 37, p. 238, 1905 

 Molisch. — Leuchtende Pflanzen, Jena, 1904. 

 October, 1911 



