Dispersion in Relation to the Electron Theory. 239 

 Experimental. 



The method adopted is the same in general principle as 

 that already used by several experimenters *, and consists of 

 the use of a half-shade polarimeter in conjunction with a 

 spectroscope, which may be placed so as to disperse the light 

 either before or after the transmission through the polarizing 

 prisms. In the former case the spectroscope takes the form 

 ■of a monochromatic illuminator. As the theory of the half- 

 shade method has been discussed elsewhere f, remarks here 

 may be confined to a description of the disposition of the 

 apparatus which has been found most suitable for measure- 

 ments in the visible and ultraviolet. 



The light from the source S (fig. 1) passes first through a 

 quartz-condensing lens of 20 cm. focal-length, and is con- 

 centrated by this on the aperture in the electromagnet, the 

 ultraviolet rays coming to foci between this aperture and 

 the polarizing prisms P. After traversing the substance in 

 T and the analyser A, the rays are collected by a quartz- 

 fluorite lens QF which forms a sharp image of the dividing- 

 line of the polarizer on the slit V of a wave-length spectro- 

 meter of the constant- deviation type. The rays are deflected 

 to this slit by a totally-reflecting glass prism which is 

 mounted so that it can be racked down out of the path of 



□ 



Fig. 1. 



O— 



&'-.—{ 



U. 



F-- 



the 



light 



when required. In this case the light falls 

 directly on the slit of a quartz- spectrograph SP. With this 

 arrangement the condition of the illumination could be 

 tested at any time during a series of photographic exposures, 

 a great convenience when using sources of variable intensity. 

 As the prism of the constant- deviation spectrometer absorbs 

 somewhat strongly in the violet, the visual observations 

 were not continued beyond \ = *4958yit. Observations on 



* Landau, Phys. Zeits. p. 417, 1908 ; and Lowry, Proc. Roy. Soc. 

 1908, A, p. 472. 



t Landau, he. cit, and G. J. Elias, Ann. der Phys. (35) p. 299 (1911). 



