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THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XII. — On the Cheapest Form of Light, from studies 

 at the Allegheny Observatory ; by S. P. Langley and F. 

 W. Very. (With Plates III, IV and V.) 



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 with- 

 out heat other than that in the light itself ; that this is actually 

 effected now by nature's processes ; 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 purpose is nearly synonymous with heat ; but 

 as a given amount of heat is producible by a known expendi- 

 ture of fuel at a known cost, the word " cheapest " may also 

 here be taken with little error in its ordinary economic appli- 

 cation. 



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 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 favorable 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 which we actually get by the present most widely used 



* 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 indefinitely over 99 per cent of the 

 radiant energy is (for illumination purposes] waste. 



Am. Jour. Sci. — Third Series, Vol. XL, No. 236.— August, 1890. 

 7 



