ON THE CHEAPEST FORM OF LIGHT. 



From Studies at the Allegheny Observatory.* 



BY 



S. p. LANGLEY AND F. W. VERY. 



The object of this memoir is to show by the study of the radiation of 

 the fire-fly that it is possible to produce light without heat other than 

 tliat in the light itself; that this is actually effected now by nature's 

 l)rocesses, and that these are cheaper than our industrial ones in a degree 

 hitherto unrealized. By " cheapest " is here meant the most economical 

 in energy, which for our ])urpose is nearly synonymous with heat; but 

 as a given amount of heat is producible by a known expenditure of fuel 

 at a known cost, the word " cheapest '' may also here be taken with little 

 error in its ordinary economic a})plication. 



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 illumination, where, as I have alread}^ 

 shown, it ordinarily exceeds 99 parts in the 100; and least in sources of 

 liigh temperature, like the incandescent light alid electric arc, where yet 

 it is still immense and amounts even under the most favorable condi- 

 tions to very much the larger part. 



It has elsewhere t been stated that for a given expense at least one 

 hundred times the light should in theory be obtainable which we actually 

 get by the present most widel}^ used methods of illumination. This, it 

 will be observed; is given as a minimum value, and it is the object of the 

 jiresent research to demonstrate that not only this possible increase, but 

 one still greater, is actually obtained now in certain natural jjrocesses, 

 which we know of nothing to prevent our successfully imitating. 



It is now universally admitted that wherever there is light there has 

 been ex})enditure of heat in the production of radiation 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 



* Reprinted, with additional note, from the American Journal of Science, third 

 series, vol. xl, No. 236, August, 1890. 



fSee results of an investigation by S. P. Langle}', 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 indefinitely over 99 per cent of the radiant 

 energy is (for illumination purposes) waste. 



(••5) 



