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_. [Illustration: FIG. 15.--Spectral energy, luminosity and visibility curves (_after Gibson and McNicholas_) A. Spectral energy curve of Hefner lamp. B. Spectral energy curve of acetylene flame. C. Spectral energy curve of tungsten (gas-filled) glow lamp. D. Spectral energy curve of black body at 5000° absolute (sunlight). E. Spectral energy curve of blue sky. H_g_. Spectral energy curve of Heræus quartz mercury lamp. L_v_. Visibility curve for human eye. L_a_. Luminosity of Hefner lamp. L_e_. Luminosity of blue sky. ] The luminous efficiencies of various forms of artificial illuminants have been calculated by Ives (1915) and are given together with that of the firefly in Table 6. Fig. 15 gives spectral energy curves for various illuminants reduced to 100 at λ = .590µ, luminosity curves for the Hefner lamp and blue sky, and a visibility curve worked out by Coblentz and Emerson (1917) from observations on 130 individuals. TABLE 6 _Luminous Efficiencies of Various Illuminants_