( 613 ) 



lenses L^ , L^ niul L.^ , />„ and the totally-reflecting prism Pr to 

 an image of' tlie l)eam of light crossing the ethylene tul)e on the slit 

 of the spectrosco|)e >S'/> (a direct-vision spectroscope of Hilgek-Christif, 

 giving a spectrnm of great intensity^), in which an eye-piece slit has 

 been made in order (o confine a certain |)ortion of the s|)ectrnm ; 

 by means of the screw -S'c''', different |)ortions of the spectrnm may 

 be l)ronght into the field). A t)eam of the light emitted by the Nernst- 

 lamp is thrown on the slit of the spectroscope by means of plane 

 mirrors through the polarizing prisms N^lc\, Nic^, A^c, (after having 

 been first made parallel by lens L.) and then through lens L^ and a 

 totally-reflecting prism. The prisms iV7ci and Nk\ are rigid, which 

 ensures that the light thrown into the spectroscoj)e bj^ the reflections 

 on the mirrors with different positions of Xic.^ is reduced in the 

 same proportion; the prism jVic^ can turn round, and is provided 

 with a graduated circle, which could be read up to 3'. The plane 

 of polarisation of A7c,, is horizontal so that the condition of polai-i- 

 sation in (he two l)eams thrown into the spectroscope agrees in the 

 main ^). The plane of polarisation of Nk\ has been put parallel to 

 that of yic\. After the tube of ethylene had been brought to the 

 required temperature, and the tem[)ei"ature of the room had i)een 

 regulated in such a way that the temperature of the tube of ethylene 

 (read to 0^.01) could be kept sufficiently constant (up to some hund- 

 redths of a degree) by the addition, when necessarv, of some cold 

 or hot water into the vacuum glasts, tiie prism Xic^ was adjusted 

 by rotation so as to obtain ecpuxl intensity of the considered portions 



1) See Zeeman (Jonnn. N". 5, June '9:2, more detailed Arch. Néerl. 27 (1893) 

 p. 259 and PI. V. The "lialfprism" was used in our experiments with a view to 

 the intensity in the magnifying position (Chkistik, Proc. Roy. Soc. 26 (1877) p. 8). 

 Moreover the dispersion is greater in this position, wliereas tiie loss of purity in 

 the spectrum is of no importance here. 



~i Not too near tlie critical state tlie light emitted by the blue mist in a 

 direction normal to the incident light, is polarized in the plane of incidence 

 (Ramsay, ZS. physik. Uhem. 14 (1894) p. 480). It is to be expected tlial on 

 approach to the critical slate the light emitted in the direction mentioned becomes 

 more and more partially polarized (cf. Tvndall, Phil. Trans. 16(J (1870) p. 348). 

 It would be interesting to examine if then Tyndall's residual blue (I. c.) could 

 be observed (on the connection of this with the difference in relractivity of the 

 scattering particles and the surrounding medium see Ray'LEIGH, Phil. Mag. (4) 

 41 (1871) p. 454). Measurements on the condition oi' polarisation might also 

 lead to an opinion on the size of the particles, see Book, Wied. Ann. 68 (1899) 

 p. 674 (spectrophotometrical investigation of the light scattered by a jet of steam, 

 measurement of the condition of polarisation, and determination of the size ol' the 

 particles by means of ditlraclion rings) and Pernter, Denkschr. Kais. Ak. d. Wiss. 

 VVien 73 (1901) p. 301. 



