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 physiologic light to analysis by the spectroscope, and with a few exceptions the results have agreed very well. The best known of these spectroscopic 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, structureless band" in the red, yellow and green, but not extending further than wave length toward the red end of the spectrum, nor than wave length 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 extremely high luminous or radiant efficiency of the light. This was estimated 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 produces the greatest amount of luminous radiation.
But even if we should discover the means by which the firefly produces its light, we should hardly care to use it in our homes. The insect 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.
