Cheapest Form of Light. 261 



"cheapest" may also here be taken with little error in its 

 ordinary economic application. 



We recall 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-illumina- 

 tion, where, as I have already shown, it ordinarily exceeds 99 

 parts in the 100 ; 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 favourable 

 conditions, to very much the larger part. 



It has elsewhere * been stated that for a given expense at 

 least one hundred times the light should in theory be obtain- 

 able that we actually get by the present most widely used 

 methods of illumination. This, it will be observed, is given as 

 a minimum value, and it is the object of the present research 

 to demonstrate that not only this possible increase, but one 

 still greater, is actually obtained now in certain natural pro- 

 cesses, which we know of nothing to prevent our successfully 

 imitating. 



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 

 slightest visible result, while each increase of the light repre- 

 sents not only the small amount of heat directly concerned in 

 the making of the light itself, but a new indirect expenditure 

 in the production of invisible calorific rays. Our eyes recog- 

 nize heat mainly as it is conveyed in certain rapid sethereal 

 vibrations associated with high temperatures, while we have no 

 usual way of reaching these high temperatures without passing 

 through the intermediate low ones ; so that if the vocal produc- 

 tion of a short atmospheric vibration were subject to analogous 

 conditions, a high note could never be produced until we had 

 passed through the whole gamut, from discontinuous sounds 

 below the lowest bass, up successively through every lower 

 note of the scale till the desired alto was attained. 



There are certain phenomena, long investigated, yet little 



* See results of an investigation by S. P. Langley, read before the 

 National Academy in 1883, and given in ' Science ' for June 1, 1883, 

 where it is shown that in the ordinary Argand-burner gas-flame indefi- 

 nitely over 99 per cent, of the radiant energy is (for illumination purposes) 

 waste. 



