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LIV. On a new Revolving Polariscope. 

 By William Spottiswoode, M.A., F.R.S.* 



THIS instrument consists of a NicoFs prism or other ordi- 

 nary polarizer, and a double-image prism as analyzer. The 

 latter is so cut as to show one image in the centre of the field of 

 view, the other excentric ; and the peculiarity of the arrangement 

 consists in giving to the analyzer a rapid motion of rotation. If the 

 speed attains eight or ten revolutions per second, the image will 

 remain persistently upon the retina during an entire revolution, 

 and all the phenomena which are usually seen in succession will 

 appear displayed simultaneously in a circle or ring by the ex- 

 centric image. 



The central image will consist of a superposition of the images 

 due to all the successive azimuths of the analyzer, and will con- 

 sequently appear unchanged in brightness or in colour during 

 the working of the instrument. 



In particular, if the polarizer and analyzer be used without 

 any interposed plate, the excentric image will as usual be bright- 

 est at two positions opposite to each other, say at 0° and 180°, 

 and dark at the two positions 90° and 270°. A rapid revolution 

 of the analyzer will therefore give the appearance of a ring 

 brightest at the two positions first mentioned, and fading into 

 darkness at the two other positions. 



If a plate of selenite be interposed with its axis at 45° to the 

 original plane of polarization, the two images will present com- 

 plementary tints at 0° and 180°, and likewise at 90° and 270°; 

 but at the two latter positions the tints will be the reverse of 

 those at the two former. At intermediate positions the tints 

 will be fainter, while 45° and 135° will be positions at which each 

 tint is passing into its complementary, and all colour is lost. 

 In this case, therefore, the ring will appear coloured in opposite 

 quadrants with the same tint, in the intermediate quadrants 

 with the complementary tint. In the intervening parts the 

 tints fade into one another. 



If a plate of quartz cut perpendicularly to the axis be used 

 instead of the selenite, the entire series of spectral tints will be 

 seen displayed twice over in the ring. The order of the tints 

 in the ring will for a given direction of revolution depend upon 

 the character of the quartz, i. e. whether it is positive or negative. 



Some interesting experiments may also be made with a quarter- 

 undulation plate. By this means plane polarization may, as is 

 well known, be converted into circular, and circular into plane. 

 Hence, if we place a quarter-undulation plate in front of a sele- 

 nite, we shall produce the complete series of tints in the ring, as 



* Communicated by the Author 



