S. P. Langley — Energy and Vision. 359 



Art. XXXIX. — Energy and Vision ;* by S. P. Langley. 



While it is quite a familiar fact that the luminosity of any 

 spectral ray increases proportionately to the heat in this ray, 

 and indeed is but another manifestation of the same energy, I 

 have recently had occasion to notice that there is, on the part of 

 some physicists, a failure to recognize how totally different op- 

 tical effects may be produced by one and the same amount of 

 energy according to the wave-length in which this energy is 

 exhibited. 



I should not perhaps have thought it advisable to make this 

 last remark, were it not that there has appeared in a recent 

 number of Wiedemann's Annalen, a paper by H. F. Weber on 

 " The Emission of Light," in which he tacitly makes the assump- 

 tion that the luminosity of a color is proportionate to the ener- 

 gy which produces it, an assumption which it is surprising to 

 find in a paper of such general merit and interest. 



In another article of the same number of the journal, the 

 mistake was pointed out by Professor F. Stenger, who remarked 

 that Mr. Weber's assumption was inconsistent with the investi- 

 gations of the present writer. Still the fact that there could 

 be such a misapprehension at the present day, led me to look 

 at the matter again, and to observe, with some surprise, that 

 there was nowhere, in any physical work known to me, any ex- 

 act or even approximately exact statement of the relative ocular 

 effects of a given amount of energy in different parts of the 

 spectrum. I have undertaken therefore during the last few 

 months an experimental re -investigation of this subject, with 

 such a statement especially in view. 



We shall evidently need two correlated sets of experiments, 

 the first set to determine the amount of energy in each ray, the 

 second to show the corresponding visual effect. 



For the first of these, since energy only shows itself through 

 absorptive media which more or less disguise it, we must select 

 that manifestation which disguises it least, and in this respect 

 beyond comparison the thermal one stands first, as the heat dis- 

 persed by a glass prism and shown by a thickly lampblacked ther- 

 mometric apparatus is, throughout the visible spectrum, very 

 nearly proportionate to the energy itself. For these first or 

 thermal experiments, whence the energy is readily deduced, 

 with close approximation, we shall rely principally upon a very 

 elaborate investigation made here some time since and already 

 published, where the bolometer is used to deduce in terms of 

 lampblack absorption the relative amounts of solar energy in 



*Read by abstract before National Academy of Sciences, April 19, 1888. 



