Fig. 14.—Luminous efficiency of the firefly, shaded area ÷ total area (after Ives and Coblentz).
The spectral energy curve for the firefly has been worked out by Ives and Coblentz (1910), using a photographic method in which the intensities of different wave-lengths of the firefly (Photinus pyralis) light is com
pared with that of a carbon glow lamp by measuring the amount of photochemical change produced on panchromatic photographic plates. [Fig. 11] gives the energy curves of various fireflies and the carbon glow lamp in the same spectral region. The visual sensibility curve used by Ives and Coblentz is that of Nutting (1908, 1911), based on Konig's data. It is reproduced in [Fig. 6]. The latest visibility curve is that of Hyde, Forsyth and Cady (1918), reproduced in [Fig. 12]. It is based on observations of twenty-nine individuals. As individuals vary considerably in their sensibility to different wave-lengths, the visibility curve represents an average, but it is the only standard we have with which to evaluate the energy we call light. Color-blind individuals would have a visibility curve very different from normal individuals. Composite curves showing the luminous efficiency of the 4-watt carbon glow lamp and the firefly, both in relation to visibility, are given in Figs. [13] and [14], respectively. In these figures
the luminous efficiency is the shaded area ÷ total area, 0.43 per cent. for the carbon glow lamp and 99.5 per cent. for the firefly, "these numbers representing the relative amounts of light (measured on a photometer) for equal amounts of radiated energy—a striking illustration of the wastefulness of artificial methods of light production. From the specific consumption of the tungsten lamp (1.6 watts per spherical candle) and the mercury arc (.55 watts per spherical candle) we obtained by comparison with the carbon filament that their luminous efficiencies are 1.3 and 3.8 per cent. The most efficient artificial illuminant therefore has about 4 per cent. of the luminous efficiency of the
firefly." This is calculated to be .02 watts per candle. More recent determinations (Coblentz, 1912), using a new sensibility curve of Nutting's (1911) for a partially light-adapted eye, give the reduced luminous efficiency as 87 per cent. for Photinus pyralis, 80 per cent. for Photinus consanguineus and 92 per cent. for Photuris pennsylvanica.
