former consisted of radiant heat shorter than λ = 3µ and extending up to the visible light rays (λ = 0.7µ since the bunsen flame emitted no light). The very slight effect of the _Pyrophorus_ radiation must be due to wave-lengths between λ = 3µ and λ = 0.468µ, the limit of the _Pyrophorus_ spectrum in the blue. Langley and Very assumed it to be due entirely to the band of visible light, λ = 0.640µ to λ = 0.468µ, and assumed that no invisible heat rays were produced. All of the energy of _Pyrophorus_ light would therefore lie in the visible region and its efficiency (light rays ÷ heat + light + actinic rays) would be 100 per cent. Later, Langley (1902) reinvestigated the radiation of _Pyrophorus_ and could detect no heating whatever with the bolometer. "A portion of the flame of a standard sperm candle, equal in area to the bright part of the insects, gave under the same circumstances, a bolometric effect of such magnitude that had the heat of the insect been 1/80,000 as great as that from the candle, it would certainly have been recognized." Coblentz (1912) also, using a vacuum thermopile of Pt and Bi, was unable to detect any infra-red radiation from _Photinus pyralis_, but found that the temperature of this firefly is slightly lower than the air. These temperature measurements will be discussed in a later chapter. [2] Langley and Very evidently supposed that the body temperature of the firefly, like the mammal or bird, is higher than its surroundings. The assumption of Langley and Very that the small amount of _Pyrophorus_ radiation passing glass is all light has been called into question by Ives (1910), who points out that Langley and Very failed to use a screen which would cut off either the visible rays or the invisible rays between 3µ and 0.7µ. They really left the question open as to whether the effect of _Pyrophorus_ light on their bolometer was due to the visible band of rays or to this plus another band in the infra-red. "The firefly's actual efficiency as a light source is dependent to a large degree on the radiation being confined to the visible region. If there should be found infra-red of quantity comparable to the visible, the firefly, while still a very efficient source would not be, as usually supposed, the example of an ideally efficient light produced by nature."