Io8 BIOLOGICAL LECTURES. 



organs of vision, produce Jiardly any thermal or actinic effect. In 

 other words, very little of the energy expended in the fash of 

 the fire-fly is ivasted. It is quite different with our artificial 

 methods of illumination. In the case of an ordinary gaslight 

 the best experiments show that not more than one or two per 

 cent of the radiant energy consists of visible rays, the rest is 

 either invisible heat or actinism ; that is to say, over ninety- 

 eight per cent of the gas is wasted in producing rays that do 

 not help in making objects visible." 



Of Professor Langley and Mr. Very's more recent and well- 

 known paper "On the Cheapest Form of Light," ^ most of you are 

 doubtless aware. As their observations are the most accurate 

 extant on the physical properties of animal light, it will not be 

 out of place to reproduce here at length the essential points of 

 their conclusions, as well as some of their instructive state- 

 ments on the concepts of radiant energy, which underly their 

 experimental inquiries. 



"We recall," says Professor Langley, "that in all industrial 

 methods of producing light, there is involved an enormous 

 waste, greatest in sources of low temperature like the candle, 

 lamp, or even gas illumination where, as I have already shown, it 

 ordinarily exceeds ninety-nine parts in the one hundred ; and 

 least in sources of high temperature like the incandescent light 

 and electric arc, where yet it is still immense and amounts, even 

 under the most favorable conditions, to very much the larger 

 part" (p. 97). 



" It is now universally admitted that wherever there is light, 

 there has been expenditure of heat in the production of radia- 

 tion existing in and as the luminosity itself, since both are but 

 forms of the same energy; but this visible radiant heat which 

 is inevitably necessary is not to be considered as waste. The 

 waste comes from the present necessity of expending a great 

 deal of heat in invisible forms before reaching even the slight- 

 est visible result, while each increase of the light represents not 

 only the small amount of heat directly concerned in the making 



1 S. p. Langley and F. W. Very : On the Cheapest Form of Light, from Studies 

 at the Allegheny Observatory with Plates IIL IV, and V. The American Journal 

 of Science, Third Series, Vol. XL, No. 236, August, 1S90. 



