4 A PHYSICAL STUDY OF THE FIREFLY. 
of their lighting systems is no higher than our own (but their fuel supply is 
evidently different from ours) and to learn that much of theirsecret may be 
of value. 
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 
anactual 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. 
While much has already been done on the light of the firefly, much more 
remains undone, all of which no doubt will have direct bearing on the manner 
of light production in animals. Among the important problems awaiting 
solution are (1) whether, in animals having luminous organs emitting light 
of different colors, the maxima of emission are different; and (2) whether 
the maximum emission in any one photogenic cell is the same when the 
light is emitted from the living animal and when the photogenic material is 
excited to light emission by an oxidizing agent after the luminous organs 
have been dried. 
Heretofore we have been led to think that “light”? must be accompanied 
by infra-red radiations; and we seem utterly unable to turn away from this 
idea. The various investigations of the firefly have without doubt opened 
up a new line of thought in regard to the emission of heat and light. 
*Knab., Canadian Entomologist, 37, p. 238, 1905. Molisch, Leuchtende Faancen, 
Jena, 1904. 
