22 



THE NATURE OF ANIMAL LIGHT 



The total radiant energy which a body emits is a func- 

 tion of its temperature and for a perfect radiator, or what 

 is known as a black body, the total radiation varies as the 

 ^fourth power of the absolute temperature, T. (Stefan- 

 Boltzmann Law). The radiant energy emitted at different 

 wave-lengths is not the same but naore energy is emitted 

 at one particular wave-length (\max,) than at longer or 

 shorter ones, depending also on the temperature. If the 

 various waves are intercepted in some way, their relative 

 energy can be measured by an appropriate instrument 

 and spectral energy curves can be drawn, showing the 



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X RAYS . MPLORED 



HERTZIAN 

 ELECTRIC 



Fia. 1. — Schematic representation of various types of radiation to form a wide continuous 



spectrum. 



distribution of energy throughout the spectrum. Fig. 2 

 gives a few of the curves, and it will be noted that the 

 maximum shifts toward the shorter waves the higher the 

 temperature. In fact, for a black body Ama^. x T = 2890, 

 and at 5000° C. (about the temperature of the sun) Amax 

 lies within the visible spectrum. In gas or electric lights it 

 lies in the infra-red region. The area enclosed by these 

 spectral energy curves represents the total energy emitted, 

 and, knowing this and the area enclosed by the curve of 

 visible radiation, it is easy to determine how efficient a 

 source of light is as a light-producing body. We shall 

 inquire more fully into this question in Chapter III, in con- 

 sidering the efficiency of the firefly as a source of light. 



