Cheapest Form of Light. 265 



and thus differ from ignited solids and liquids, which have 

 continuous 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 illumi- 

 nation. 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 f . 



Secchi% at first thought that the spectrum of the glow- 

 worm was monochromatic, but, with an improved spectroscope, 

 recognized that other colours were present, though feebly, and 

 decided that the spectrum was sensibly continuous. 



Quatrefages§, in connexion with the paper of Secchi, 

 remarks that the previous observations of Spallanzani and 

 Macaire, repeated with much care by Matteucci and Becquerel, 

 show beyond doubt that the light of glow-worms and elaters 

 is due to slow combustion. Thus the light is extinguished in 

 a vacuum and in irrespirable gases, it reappears in contact 

 with the air, it is perceptibly increased by the presence of pure 

 oxygen, it persists after the death of the creature, and finally 

 it is accompanied by the generation of carbon dioxide. 

 Nevertheless he points out that there is a distinct kind of 

 phosphorescence in the marine Noctilucidw, due to the con- 



* Tlie American Naturalist, Salem, 1870, vol. iii. p. 615. 

 i S. P. Langley lias shown that the waste is in fact even greater than 

 this ; see ' Science/ vol. i. No. 17, p. 482 (1883). 



t Comptes Rendu* , lxxv. p. 321 (1872). § Ibid. p. 322 (1872). 



