774 
magnetic rotation in the manner indicated by Smrtsema’), the question 
arises as to whether ultraviolet measurements would not enable one 
to ascertain if this value of €/„ is actually constant, and thus justify 
such a simple assumption for the case of hydrogen. As the ultra- 
violet magnetic rotation has hitherto been investigated only for solids 
and liquids?), I was glad to accept the invitation extended to me 
by Prof. KAMERLINGH ONNES to endeavour to extend SIERTSEMA's *) 
measurements to the ultraviolet region of the spectrum with the 
same apparatus as the latter had used. 
2. In order to obtain good results from the use of this apparatus 
absorption of the ultraviolet rays had to be prevented, hence quartz 
was chosen instead of glass as the material for the covers, lenses and 
prism, while the canadabalsam-nicols were replaced by GTAN's air 
layer nicols. 
For preliminary experiments I used a fluorescent eyepiece filled 
with aesculin solution, but this was found unsuitable on account of 
the small intensity of the light. When I had a spectrograph at my 
disposal I was able to photograph the dark rotationband which 
occurs in the Brocn-WieDEMANN method and which Srertsema had 
used for purposes of adjustment, but in the ultraviolet the band 
was too broad and the spectrum was too feeble to allow the centre 
to be determined to the desired degree of accuracy. I, decided there- 
fore to follow Lanpav and use a half-shadow method. 
In this method a half-shadow analyser divides the field into two 
halves, whose planes of polarisation make a small angle of 24° with 
each other; if now a rotating substance is placed in the path of 
the rays between the nicols, and the rays from the analyser are 
received in a spectroscope, two spectra are formed, one above 
the other, in which the dark rotationband does not occupy the 
same position. If the angle of rotation for the position of the band 
in the one spectrum is @°, then in the other spectrum the band is 
at a place where the rotation is («+ 2d)°. At a point at which the 
rotation ‘is (a + d)°, there is, for a special wavelength, the same 
intensity in the two spectra. On rotating the polariser the position 
1) SIERTSEMA, These Proc. Vol. V, p. 413. 
2) van ScHAIK, Proefschrift Utrecht 1882. JouBiN, Ann. Chim. Phys. S. 6, T.6, 
p. 78. 1889. BoreL, Arch. des Sc. Phys. et Nat. Genève, 16, p. 24, 1903. LANDAU 
Phys. Ztschr. 9, p. 417, 1908. Darmois, Ann. Chim. Phys. 5. 6, T. 22, p. 247, 
495, 1911. 
3) SIERTSEMA, Versl. K. Ak. v. Wet, 24 Juni 1893, p. 31; 26 Jan. 1895, p. 230; 
28 Maart 1896, p. 294; 24 December 1898, p. 280; 27 Mei 1899, p. 4, 
