THE LUMINOUS EFFICIENCY OF ILLUMINANTS 539 



ture and a corresponding gain in luminous efficiency ; at the 

 same time, the lights have become whiter in tint. Thus metallic- 

 filament lamps yield a whiter light than the old carbon filament ; 

 if we consider the past, we find light sources of a still ruddier 

 hue in the gas flame, the petroleum lamp and the pine-wood 

 torch of the Middle Ages. 



It will be observed that the light from all the sources 

 mentioned is derived from incandescent solid particles and 

 the nature of their radiation is too nearly akin to that of [a 

 black body to constitute them highly efficient illuminants. In 

 Fig. i the writer has plotted the energy curves of the sun 

 and of several artificial illuminants and also the well-known 

 luminosity curve for the eye throughout the spectrum. The 

 maximum sensitiveness of the eye is located, it will be observed, 



o - 3 o'4 o*s 0"6 o'7 o - 8 0*9 fo i'i I"2 1*3 1*4 1*5 1*6 l'7 l"8/i 



Fig. 1. — Showing distribution of energy in spectra of sunlight, acetylene and arc light, 



and sensitiveness of eye. 



in the yellow-green at about 0*5 5//.. The maxima of the energy 

 curves of the artificial illuminants fall far without the visible 

 spectrum and the proportion of visible light produced is 

 correspondingly minute. But it is most interesting to observe 

 that the energy curve of the sun has its maximum very near 

 that in the luminosity curve of the human eye. This sug- 

 gests that our vision has been so developed as to make the 

 best possible use of natural light. Presumably if the maximum 

 of this energy curve had been located in the blue, our eyes 

 would have so developed as to be most sensitive to light of 

 this colour and our perception of colours in general would 

 have been radically different. 



In passing it may be pointed out that this raises one very 

 interesting question, by which the validity of this theory might 



35 



