42 A PHYSICAL STUDY OF THE IflRElflvY. 



it seems just as probable that the explanation will be found in biochemical 

 phenomena not requiring the production of radiations of low frequency in 

 order to produce a given amount of light. Set as we are in our ways of 

 thinking, we insist that the methods employed by the firefly to produce its 

 light must be similar to our own. We might as legitimately suppose that 

 the combustion of food in our bodies, with the resultant formation of CO2 

 and H2O, or that the combustion in a growing leaf, must be accompanied by 

 a rise in temperature and emission of radiant energy (other than ' ' animal 

 heat") simply because we observe all these phenomena when we light a 

 candle or heave coal into the fire. 



In recent papers* the writer has published numerous illustrations showing 

 that in incandescent solids and in gases excited in vacuum tubes the produc- 

 tion of light is accompanied by a very considerable amount of infra-red 

 radiation; that in the arc light, especially the "flaming arc" vapors, there 

 is little infra-red radiation ; and that in the high-frequency spark nearly all 

 the radiation is in the ultra-violet. The light of the firefly is not unlike that 

 of an ideal radiator at a temperature of 5,000°, differing from it only in the 

 density of the radiation. If the specific emissivity of the sun were as low as 

 that of the firefly, it is quite possible that no infra-red radiation could be 

 detected. The solar corona is another example of this type. Perhaps the 

 question may become clearer when we can formulate some ideas on the 

 emissivity from a selectively absorbing substance in the region of a band of 

 anomalous dispersion. The ideal radiator is one having no reflectivity. 

 Only in the region of anomalous dispersion, where the refractive index 

 becomes practically nil, is this condition fulfilled, as far as the reflecting 

 power is concerned. 



*Jahrb. der Radioaktivitat und Electronik, vii, p. 123, 1910; Illuminating Engineer, 

 London, i, p. i, 1910. 



