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Physics. — "Ou the duuhlc refrdctiuii, in a nnft/iK'tic fichl ih'ar tlw 

 coitqjonenls of a quadnqjkL" llj Prof. P. Zkkman and .1. f4KKsT. 



On a former occasion llio resulls wci-e coininiinicalcd to ilic 

 Academy, of an invcstig-atioji on lli(> mauiiolic rotation of the plane 

 of i)olanzation in sodinm va}X)nr, jji liie immediate nei<;lil>oiirliood 

 of (lie absorption lines. ^) 



In tiie case of very thin va|)onrs this rotation appeared to he 

 positive ontside tiie com])onents of the donhlet, in \vliich the original 

 spectral line is resolved by the intlnence of the magnetic forces; 

 between the components, however, it becomes negative and \erv 

 large. In these experiments the light of conrse passed throiiuh the 

 vai)onr in the direction of the lines of force. 



In the same way, if the light is transmitted throngh sodinm \aponr 

 in a direction normal to the lines of force, we may exj)cct from the 

 examination of the immediate neighbonrhood of the com[)onents, 

 in which the S})ectral line is split n[) by the magnetic forces, resnlts 

 which are of theoretical importance. 



VoiGT has deduced from his theory of magnet()-o[)tical plieiiomeiia 

 the existence of a double refraction, which must be j)ro(lnce(l in 

 isotropic media, as soon as they are placed in a magnetic field, but 

 which should only l)e observable in the neighbonrhood of an absoi-p- 

 tion line.^) Voiot, together with Wiechkut, has obsei-\ed, that i)lan<> 

 l)olarised light of a period near that of the lines I)^ and /J.,, is no 

 longer plane polarised but has become elliptically i)()Iarise<l when 

 it has traversed the tlanie, there being generated a ditference of phase 

 between the com[)onents \ibrating parallel and those \ibrating per- 

 pendicularly to the field. 



This elliptical polarisation \vas demonstrated by the above mentioned 

 physicians with the aid of a Bahinet compensator, using a flame 

 with much sodium and a small Rowland gratiiig. 



The ol)ject of our investigation of the magnetic double refraction 

 was to examine the phenomena, which show themselves, if, beginning 

 with very small vapour densities, the quantity of sodium is gradually 

 increased. The [)resent communication deals oidy with the line J)^ 

 in the case of very small densities. This line is resolved into a 

 quadruplet by the action of the magnetic Held. 



The grating em[)loyed for this investigation and its mounting foi- 



1) Zeeman. Proc. Hoy. Acad. Am.steiiUun Vol. V p. il, 1902, cf. als(j Hallo 

 Dissei-tatie. Amstei-dani, 1002. 



-) VoiGT. Götlingei- iXacliricliteu. Hell i. 18'.)8; W'ilül.ma.nn'ö Annalen, lid. (.17. 

 p. 359, 18'J'J. 



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