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. f 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 Pholuris pennsylvanica is about 92 per cent. The 

 latter emits the bluer hght, corresponding more nearly with that part of the 



spectrum for which the eye has its maximum 

 sensibility (at 0.54^1) and hence should be the 

 more efficient. 



The higher value, 96.5 per cent, previously 

 obtained for the Pholinus 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 

 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 muchj 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. 



Fig. 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, 191 1. 

 Circular Bur. Standards 28, p. 8. 



fBull. Bur. Standards, vol. 6, No. 3, p. 331. 

 jives, Trans. Ilium. Eng. Soc, 6, p. 258, 191 1. 



