Langley and Very — Cheapest Form of Light. 101 



of oxygen with carbon which is one of the elements of the 

 phosphorescent matter. The greatest brilliancy occurred at a 

 temperature of 37° or 38° Cent., but all phosphorescence ceased 

 above 50° or below — 6° Cent. 



Robert x found that a glow-worm cut in halves continued to 

 glow for half an hour, when the light ceased, but commenced 

 again on the near approach of a candle, and continued as bright 

 as ever for thirty-six hours, after which it was impossible to 

 renew it. 



Pasteur* has examined the spectra of our Pyrophorus with- 

 out finding any appearance of bright or dark lines. He states 

 that M. Gernez has made a similar observation on the spec- 

 trum of the glow-worm. 



BecquereV gives a good summary of the results of previous 

 observers. Since phosphorescent solids give banded spectra 

 and thus differ from ignited solids and liquids which have con- 

 tinuous spectra, M. Becquerel concludes, from the apparent 

 continuity of the spectrum of the light from phosphorescent 

 animals, that their light approaches nearer to that of ordinary 

 incandescence, — a deduction which the following result renders 

 unnecessary. 



C. A. Young* states that the " common " fire-fly gives a 

 continuous spectrum, extending from a little above Fraun- 

 hofer's line C in the scarlet, to about F in the blue, gradually 

 fading out at the extremities. He observes that it is notice- 

 able that precisely this portion of the spectrum is composed of 

 rays, which, while they more powerfully than any others 

 affect the organs of vision, produce hardly any thermal or 

 actinic effect. In other words very little of the energy ex- 

 pended in the flash of the fire-fly is wasted. 



(This is a most important and interesting inference, but it 

 will be observed that this is necessarily rather assumed as 

 highly probable than actually demonstrated, since the method 

 did not permit the dealing with the invisible rays except by 

 inference.) 



It is quite different with our artificial methods of illumina- 

 tion. In the case of an ordinary gas light, experiments show 

 that at most, one per cent of the radiant energy consists of 

 visible rays, the rest being invisible heat ; that is to say over 

 ninety-nine per cent of the gas is wasted in producing rays 

 that do not help in making objects visible. 6 



1 C. R., xvii, p. 627, 1843. 



2 C. R., lis, p. 509. 1864. 



3 "La lumiere," 1867. 



4 The American Naturalist, Salem. 1870, vol. iii, p. 615. 



5 S. P. Langley has shown that the waste is in fact even greater than this ; 

 see " Science,'' vol. i, No. 17, p. 482. 1883. 



