THE LUMINOUS EFFICIENCY OF ILLUMINANTS 545 



the visual purple. Whatever the true explanation may be, 

 however, it seems clear that the quality of light which gives 

 the best results with fairly high illuminations of say 1-10 foot- 

 candles, such as occur in everyday practice, may be very 

 different from that which it is desirable to use when very faint 

 lights are concerned. For example, yellow monochromatic light 

 might prove the most efficient for brilliantly illuminating an 

 advertisement placard. But for marine signals, the value of 

 which is mainly judged by the limiting distance at which the 

 light can just be seen as a luminous point, greenish-blue light 

 might be preferable. 



The great majority of industrial uses of light, however, 

 demand a fairly high illumination and should therefore be 

 judged on what may be termed the upper register of vision. 

 Acting on this assumption, P. G. Nutting has recently calcu- 

 lated that the most efficient light for creating brightness is that 

 of wave-length 0'54/u-. If it were possible to secure a source 

 which produced this quality of light and this alone, he estimates 

 that theoretically an efficiency of as much as 65 C.P. per watt 

 should be possible. 



Needless to say, we have no commercial sources of light 

 at present which even approach this degree of efficiency. As 

 explained above, even the most powerful flame-arc lamps, in 

 which correctness of colour has been sacrificed to some extent 

 to brightness, probably do not yield more than 4-5 C.P. 

 per watt ; were a corresponding estimate of the luminous 

 efficiency of high-pressure gas lamps to be made, it is probable 

 that the luminous efficiency would not reach this value. Indeed 

 the comparatively low luminous efficiency of incandescent gas 

 lamps is a somewhat hopeful augury for the future. It leads 

 us to believe that the practical limit of possible improvement 

 by using the waste heat of combustion more scientifically is 

 still far distant. Seeing, therefore, that these lamps are already 

 among the cheapest in existence as producers of light, it seems 

 conceivable that in the future they might enable us to generate 

 light on a scale far greater than that realised at present. 



Incandescent gas lamps, it may be mentioned, yield a con- 

 tinuous spectrum, so that the further additional means of 

 securing improved luminous efficiency — specialisation in yellow- 

 green light — has yet to be applied. This principle has, how- 

 ever, already been used to some extent in artificial illuminants. 



