An Experimental Research "on some Standards of Light. 469 



'An Experimental Kesearch on some Standards of Light." By 

 J. E. Petavel. Communicated by Lord Rayleigh, F.R.S. 

 Received July 31, — Read November 16, 1899. 



The standards of light may be divided into two main divisions, 

 viz. : — 



1. Flame standards. 



2. Incandescent standards. 



The first class comprises such standards as depend for their constancy 

 on the rate at which chemical combination is going on. Almost all 

 the standards in actual use come under this division. The British 

 candle, the Methven standard,* the Harcourtf pentane standard, 

 the Hefner-AlteneckJ amyl acetate lamp, the Carcel lamp, and the 

 acetylene§ burner are among the best known. 



Apart from the large number of independent investigators who 

 have carried out researches as to the relative merits of these sources 

 of light, reference may be made to the reports of several committees 

 which have been appointed in this and other countries to investigate the 

 subject. 1 1 



The general conclusions reached may be fairly summed up by saying 

 that the pentane gas standard and the amyl acetate lamp are the 

 lights which, from every point of view, have been found most satis- 

 factory. Of the two, the Hefner- Alteneck lamp is the better known, 

 and has been the subject of the more complete experimental study ; it 

 may be taken as fairly representative of its class. The light emitted, 

 as in the case of all the other flame standards, is seriously affected by 

 atmospheric impurities. Liebenthal^ has shown that, if x represent the 



* < Journal of G-as Lighting,' vol. 40, p. 42, 1882. 



f See 'Brit. Assoc. Proc.,' 1877, p. 51, and 1898, p. 845 ; also 'Report of the 

 Standard of Light Committees,' as below. 



% ' Elektrotechnische Zeitschrift,' vol. 5, p. 20, 1884 ; also ' Electrical Review,' 

 vol. 42, p. 759, 1898. 



§ Proposed by Yiolle, Ch. Fery and Pessenclen (see ' Comptes Rendus,' vol. 122, 

 p. 79, 1896; also ' Comptes Rendus,' vol. 126, p. 1192, 1898). 



|| See Blondel' s Report to the Congres International des Electriciens at Geneva, 

 1896 ; Report of the Standard of Light Committee's meeting of the Institute of 

 Gas Engineers, May, 1895, < Journ. of Gas Lighting,' vol. 65, p. 1007, 1895 ; Report 

 of the Standards of Light Committee to the British Association, 1888, p. 39 ; 

 Dibden's Report to the Metropolitan Board of Works, 1885 ; G-as Institute Com- 

 mittee, 1884, and Board of Trade Committee ; Preliminary Report of the Sub-com- 

 mittee of the American Institute of Electrical Engineers, ' Transactions,' vol. 13, 

 p. 135, 1896 ; Rapport der Photometrie Com. der Vereeniging van G-asfabrikanten 

 in Nederland, ' Journ. fur G-asbeleuchtung und Wasserversorgung,' vol. 37, p. 613, 

 1894. 



% ' Elektrotechnische Zeitschrift,' 1895, vol. 16, p. 655. 



