LUMINOUS EFFICIENCY AND CANDLE-POWER MEASUREMENTS. 27 
the visibility of the radiation. In the meantime these factors have under- 
gone revision,* and in the present work the spectral energy curves given in 
Figs. 9 and 10 were multiplied by these new factors, the numerical values 
for high intensities being used. The resulting spectral energy curve, Fig. 
12, is similar to the one previously published.| The ratio of the area of 
this curve (the ratio of shaded area to the total area of the spectral energy 
_. curve), which represents the most advantageous possible distribution of light 
to the eye, to the area of the spectral energy curve of all the light emitted 
(i. e., energy radiated X visual sensibility the total radiated energy) is the 
reduced luminous efficiency. An integration of the curves thus obtained 
indicated that this reduced luminous efficiency of the Photinus consan- 
guineus is about 79 to 80 per cent, that of the Photinus pyralis is about 87 
per cent, and that of the Photuris pennsylvanica is about 92 percent. The 
latter emits the bluer light, corresponding more nearly with that part of the 
spectrum for which the eye has its maximum 
sensibility (at 0.54u) and hence should be the 
more efficient. 
The higher value, 96.5 per cent, previously 
obtained for the Photinus pyralis, would be 
decreased by using the new visual intensity 
factors. Furthermore, as already mentioned, 
the difference in the densities of the photo- 
graphic negatives was too great, and the denser 
one was too much over-exposed, to obtain a 
proper multiplying factor which would give 
the proper intensities 
of the central (highest) 
part of the curve repre- 
senting the ratios of fire- 
fly light to glow-lamp 
Sa x light. This tends to 
give too high values for 
the radiant efficiency. 
To the writer this 
method of estimating the luminous (radiant) efficiency seems misleading, 
for the reason that individual eyes vary too much{ to attempt to define 
this quantity in terms of the ‘‘average eye.’ Then, too, in the ultimate 
analysis, the question of real interest is the total cost of production and 
the total quantity of the commodity produced. ‘The aforesaid value of 
the radiant efficiency of the firefly is obtained on the assumption that 
there is no infra-red radiation emitted. It will be shown presently that 
thus far it has been impossible to detect infra-red radiation, and hence the 
high numerical values obtained for the radiant or luminous efficiency (ratio 
of ‘‘light’’ to total radiation) seem-plausible; but it is misleading if we over- 
look the question of the energy input, about which we have no information. 
q 
NW 
Fic. 12.—Luminous efficiency of firefly (shaded area to 
total area). This curve is taken from Fig. 10. 
*P, G. Nutting, Visual sensibility of the eye, Bull. Bur. Standards, 7, p. 235, 1911. 
Circular Bur. Standards 28, p. 8. 
{Bull. Bur. Standards, vol. 6, No. 3, p. 331. 
tIves, Trans. Illum. Eng. Soc., 6, p. 258, rg1I. 
