Excerpt from A Physical Study of the Firefly

Aside from this utilitarian question, there are others of equal importance. For example, we know that closely related species of fireflies emit differently colored light, variously described as blue, green, orange, etc. Is this color variation a subjective phenomenon, resulting from the variation in color sensibility of the eye with variation in intensity, * or is it an objective reality? If the latter, then what are the laws governing the light emission? It is almost too much to hope to solve this problem in the near future, but it seems possible to settle definitely the question whether or not the light differs in composition and hence is blue, green, or orange, as observed. This knowledge is obtained from the spectral energy curves of the light from various fireflies, to be described presently.

Experimenters have become so accustomed to thinking of the artificial production of light as being accompanied by a large amount of invisible radiation that no experiments would be complete without a repetition of Langley's search for infra-red radiation. Accordingly this question has also been given attention in this paper.

In View of the doubt in many investigators' minds as to whether the color of the light from the firefly is due to a variation in intensity rather than to an actual variation in composition, it seemed worth while to attempt to settle the question by making a thorough study of the light emitted by various species of this insect. Accordingly, the photographic work was continued for about four weeks, during which time 152 photographic exposures were made, on two spectrometers differing widely in dispersion, the time of exposure varying from 30 seconds to 5 hours. The time consumed in actual photography, in holding the insects on the spectrometer slit, was over 56 hours.

The histological data is introduced for clearness in discussing the present work, as well as its bearing upon the whole subject. It permits also a discussion of the plausibility of the functions assigned to certain parts of the photogenic organs as viewed from a physical standpoint and more especially from the optical properties of materials.

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