THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JANUARY 1889. 



I. 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 optical 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 

 assumption that the luminosity of a colour is proportionate 

 to the energy 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 re- 

 marked that M. Weber's assumption was inconsistent with the 



* From an advance proof communicated by the Author, to whom we 

 are also indebted for the cliches. 



Read in abstract before the National Academy of Sciences, April 19 

 1888. 



Phil Mag. S. 5. Vol. 27. No. 164. Jan. 1889. B 



