PHYSIOLOGIC LIGHT 117 



a large variety of colors, — red, blue, violet, green, etc. — but the colors 

 are in most cases pale and dim. 



Perhaps a dozen investigators have submitted some form of physio- 

 logic light to analysis by the spectroscope, and with a few exceptions 

 the results have agreed very well. The best known of these spectro- 

 scopic investigations was that of Langley and Very, in 1890. These 

 authors worked with the Cuban cucuyo; briefly, they found that 

 the prism of their spectroscope resolved the light into a narrow band 

 in the yellow and green region of the spectrum, ending somewhat 

 abruptly and showing few red or blue rays; they were unable to find 

 that the light was accompanied by any evolution of heat, such as we 

 ordinarily associate with light produced by combustion or by electric 

 heating, and hence they called the paper presenting their results " The 

 Cheapest Form of Light." This valuable research has recently been 

 confirmed by Drs. Ives and Coblentz, working in the National Bureau 

 of Standards, in Washington, and using more sensitive instruments 

 than were available to Professor Langley and his coworker. Ives and 

 Coblentz found that the light of the common firefly (Photinus pyralis 

 K.), was resolved by the spectroscope into " an unsymmetrical, struc- 

 tureless band " in the red, yellow and green, but not extending further 

 than wave length 0.67^ toward the red end of the spectrum, nor than 

 wave length 0.51/a toward the violet end. From the facts at hand it 

 seems extremely unlikely that the spectrum could be discontinuous and 

 renewed in the infra-red or ultra-violet non-visible portions of the solar 

 spectrum. 



The remarkable fact which these researches bring out is the ex- 

 tremely high luminous or radiant efficiency of the light. This was esti- 

 mated by Langley and Very at 100 per cent., and has been shown by 

 Ives and Coblentz to be about 96 per cent. In other words, 96 per cent, 

 of the total energy radiated by the firefly is exclusively illuminating 

 radiation, and does not embrace heat or other subordinate effects. This 

 is the more remarkable when it is considered that the best artificial 

 illuminant has a luminous efficiency of only 4 per cent., and most of 

 them run less than 1 per cent. Of course, this does not mean that the 

 mechanical or chemical processes resulting in the production of the 

 light have an equally high efficiency — that is quite another matter. 

 But it does mean that for a given amount of radiation, the firefly pro- 

 duces the greatest amount of luminous radiation. 



But even if we should discover the means by which the firefly pro- 

 duces its light, we should hardly care to use it in our homes. The in- 

 sect has indeed reached the highest possible radiant efficiency, but it 

 has been accomplished at a sacrifice of color variety that makes the light 

 worse for color effects than even the ghastly green of the mercury 

 vapor arc. Anything not within a very limited range of yellow and 

 green tones would appear black. 



