II. SELECTORS. 191 



882a. Original Apparatus for Rotatory Polarization, 



by Bit. College of France, Paris. 



883. Airy's Polariscope, with appliances for approximately 

 measuring the angle between the planes of polarization and analy- 

 sation, and for determining the angle between the optic axis of 

 biaxial crystals in air or in a fluid medium. Modified and arranged 

 by the contributor when a student. Rev. NicJiolas Brady, M.A. 



883a. Large Folariscope for Projection, by Ladd. 



W. Spottiswoode, F.R.S. 



A pair of Nicol's prisms, by Ladd, the first of a large size ever constructed. 

 They are furnished with a system of lenses for showing the crystal rings, as 

 well as with other contrivances for the various phenomena of polarised light. 



883b. Revolving Analyser for Folariscope, constructed 

 by Tisley and Spiller. W. Spottiswoode, F.R.S. 



A revolving analyser, consisting of a double image prism, furnished with 

 wheel-work, whereby it may be caused to revolve with such rapidity that the 

 eccentric image may remain upon the retina during a complete revolution, 

 and thus give the appearance of a ring of light. By this means all the phases 

 of polarised light as seen successively in ordinary polariscopes may be seen 

 simultaneously. The instrument is adapted to show all the phenomena of 

 chromatic polarisation, both plane and circular. An instrument for a similar 

 purpose was invented independently by Prof. Machs, of Vienna. 



883c. A portable form of Polariscope, comprising a 

 Nicol's prism, a double-image prism, a plate of tourmalin, a Savart's 

 wedge, a biquartz, and a quarter undulation plate. These various 

 parts may be used either separately or in any combinations at 

 pleasure ; and are consequently adapted either to illustrate the 

 general laws of polarised fight, or for actual observations of atmo- 

 spheric or other polarisation not involving actual measurements. 

 It will be observed that the tourmalin plate gives the opportunity 

 of using convergent as well as parallel light. The size of the 

 instrument might, however, be reduced considerably below the 

 dimensions of the specimen here exhibited. 



Mrs. W. Spottiswoode. 



883d. Large Circle for measuring the Azimuths of Ellip- 

 tic and Rotatory Polarization, and reproducing all experi- 

 ments of polarization and reflection. 



School of Pharmacy, Paris. 



883e. Arago's Polariscope. M. Lutz, Paris. 



883f. Savart's Folariscope. M. Lutz, Paris. 



883g. Tourmalin Plates. M. Lutz, Paris. 



883h. De Senarmont's Polariscope. M. Lutz, Paris. 



