POLARISED LIGHT 371 



aperture be placed with the quartz in the stage, and the 

 double image used, the two overlapping discs will always be 

 complementary. 



215. Effects of a Revolving Analyser. By employing an 

 analyser which is swiftly rotated, at the rate of 8 to 10 

 revolutions per second, as described by Mach and the late Mr. 

 W. Spottiswoode, 1 many of the preceding phenomena which 

 are ordinarily exhibited in succession during the revolution 

 of the analyser, can be made to appear on the screen simul- 

 taneously. In Mach's arrangement the light from the 

 polariser, after passing through a stage in which any plate of 

 crystal can be placed, immediately traverses an analysing 

 Nicol, close to whose face either a square aperture or a slit 

 can be attached. The analyser with its aperture occupies one 

 end of a tube revolved by a multiplying wheel, at the other 

 end of which is a prism of glass, to deviate the beam some- 

 what from the optic axis of the instrument. The effect is 

 that, on revolving the tube, what would have been a central 

 spot, by persistence of vision now becomes a ring of light on 

 the screen. In Mr. Spottiswoode's apparatus, for the Nicol 

 and deviating prism is substituted a double-image prism, 

 giving one central image and one considerably deviated, thus 

 giving both the central spot and the ring. Beyond the 

 analyser, in either construction, is a lens to focus the aperture. 

 The radial deviation in both cases is arranged to be in the 

 plane of polarisation of the analyser, and rotates with the 

 latter. 



If now we use the square aperture alone, focussed on the 

 screen, it is plain that the ring image will be bright at two 

 opposite points of the circumference, and dark at the two 



1 Mach's instrument is described in detail in Pogg. Ann. cxlvi. (1875) 

 p. 169 ; and in Miiller-Pouillet's Lehrbuch der PJiysik. Mr. Spottiswoode's 

 is described in Phil. Mag. xlix. (1875) p. 472. There can be little doubt that 

 the latter was constructed, except as to the substitution of the double-image 

 prism, upon a prior brief notice of Mach's which had appeared in the Proc. 

 Vienna Academy, Jan. 4, 1875. 



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