THE LUMINOUS EFFICIENCY OF ILLUMINANTS 549 



the same means the dress of a danseuse can be made to gleam 

 with living fire. It has even been found possible to produce in 

 the laboratory, by means of phosphorescent paints, landscapes 

 glowing with approximately natural colours. 



The possibilities of phosphorescence appeal to the lighting 

 engineer for another reason. There is good reason to believe 

 that a source of light based on this principle would be extremely 

 efficient. Prof. S. P. Langley many years ago studied the light 

 from the firefly and came to the conclusion that its luminous 

 efficiency was of the order of ioo per cent, all the recognisable 

 radiation being concentrated in the visible spectrum. Now the 

 photometric study of these faint phosphorescent substances 

 imposes special difficulties and the bolometric methods ordinarily 

 applied are excluded by the small amount of energy available. 

 One must therefore exercise some caution in drawing conclusions 

 as to their apparent high luminous efficiency. 



Two American scientists, Ives and Coblentz, have recently 

 repeated Langley 's work on the firefly by a new and ingenious 

 photographic method and were led to similar conclusions. 

 Dr. E. L. Nichols, at Cornell University, has also made 

 researches on a large number of phosphorescent materials and 

 he, too, has been struck by the fact that the energy of such 

 substances seems to be almost invariably concentrated in the 

 visible spectrum. The light, as mentioned above, was usually 

 of a greenish or blue tint but it is possible that by the judicious 

 combination of various materials an approximation to daylight 

 might eventually be obtained. 



Let us, however, now return to a point referred to earlier in 

 this article, namely the production of light of certain predeter- 

 mined colours. It has been pointed out that, if our object were 

 only to secure, as economically as possible, a light of great 

 brilliancy, we ought to aim at producing the highly luminous 

 yellow-green rays. For many purposes, e.g. in the case of 

 searchlights, the illumination of railway yards, docks, etc., such 

 a light would doubtless answer all requirements and would be 

 vastly more efficient than most of our present illuminants. In 

 foggy conditions, however, when " penetrating power " is mainly 

 desirable, it might be preferable to aim at producing light of an 

 orange-red character, since rays of long wave-length appear to 

 make their way best through minute atmospheric particles. On 

 the other hand, if we wanted a source which should be visible 



