222 SEC. 7. LIGHT. 



pendicularly to the principal section of the prism, passes through 

 the two intersecting points of the two reflections of the ellipse. 

 An index shows at the moment the position of the prism. 



882. Table Polariscope, made by the exhibitor when a 

 youth. Rev. Nicholas Brady. 



The interest of this instrument consists in showing with what simple 

 materials a student can construct a fairly useful apparatus, the divided circles 

 being common stamped protractors ; the clamping screws, teapot thumb- 

 screws, and the mountings of the lenses ordinary simple microscope frames. 



882a. Original Apparatus for Rotatory Polarization, 



by Biot. College of France, Paris. 



883. Airy's Polariscope, with appliances for approximately 

 measuring the angle between the planes of polarisation and analy- 

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

 bi-axal crystals in air or in a fluid medium. Modified and arranged 

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



88 3a. 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. Portable form of Folariscope, comprising a Nicol's 

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

 a bi-quartz, a dichroscope, and a quarter undulation plate. These 

 various parts may be used either separately or in any combi- 

 nations at pleasure ; and are consequently adapted either to illus- 

 trate the general laws of polarised light, or for actual observations 

 of atmospheric or other polarisation not involving actual measure- 

 ments. It will be observed that the tourmalin plate gives the 

 opportunity of using convergent as well as parallel light. The 

 instrument is fitted in a case 2 inches long and ^ inch in diameter, 

 but its size might be considerably reduced below the dimensions 

 of the specimen here exhibited. Mrs. W. Spottiswoode. 



