Although photographs of the spectrum of firefly (_Photinus_) light show that it extends only to the beginning of the blue, Forsyth (1910) reports ultra-violet radiation in luminous bacteria. He exposed a plate for 48 hours to the spectrum of bacterial light dispersed by a quartz prism and got a continuous band from λ = 0.50µ (the lower limit of sensitivity of the plate) to λ = 0.35µ. However, McDermott (1911 _d_) was unable to observe fluorescence of p-amino-ortho-sulpho-benzoic acid, which responds to the ultra-violet light. Molisch (1904, book) photographed bacterial and fungus light through glass and through a piece of quartz and found no difference in density on the plate. As the exposure was brief, to avoid saturation, and as the ultra-violet, which passes quartz but not glass, has a much greater action on the plate than visible light, we must conclude that ultra-violet is absent. Ives (1910) investigated the spectrum of _Photinus pyralis_, using a quartz spectroscope, and found no evidence of ultra-violet radiation, at least as far as λ = 0.216µ. It will thus be seen that the radiation from the firefly has been very carefully studied and that no waves are given off from λ = 1.5µ to λ = 0.216µ with the exception of the short band (λ = 0.67µ to λ = 0.51µ) in the visible, and it is highly probable that no radiation is given off with wave-lengths longer than λ = 1.5µ. The firefly light remains, then, 100 per cent. efficient, differing from all our artificial sources of light, the best of which does not approach this value. As Langley and Very express it in the title to their paper, it is "the cheapest form of light," not cheapest in the sense of that we can reproduce it commercially at less cost than other lights, but cheaper in the sense that it is the most economical in the energy radiated. This energy is all light and no heat. "Cold light" has actually been developed by the firefly and concerning which "we know of nothing to prevent our successfully imitating." [Illustration: FIG. 11.--Spectral energy curves of various fireflies and the carbon glow lamp (_after Coblentz_).]