30 



Mr. R. T. Glazebrook. 



[Nov. 17, 



X. " On the Refraction of Plane Polarised Light at the Surface 

 of a Uniaxal Crystal." By R. T. Glazebrook, M.A., Fellow 

 and Assistant Lecturer of Trinity College, Demonstrator in 

 the Cavendish Laboratory, Cambridge. Communicated by 

 Lord Rayleigh, M.A., F.R.S. Received October 27, 1881. 



(Abstract.) 



The paper, of which the following is an abstract, contains an 

 experimental investigation of the relation- between the plane of 

 polarisation of light falling on the surface of a crystal of Iceland spar 

 and the angles of incidence and refraction in the cases in which only 

 one refracted wave traverses the crystal. A prism was cut from a 

 piece of spar, one face of the prism coinciding almost exactly with a 

 rhombic face, and plane polarised light allowed to fall on it at a known 

 angle of incidence. The deviation of light of a definite wave-length 

 in both the ordinary and extraordinary spectrum is observed, and from 

 that and the angle of incidence we can calculate 0' and 0", the 

 two angles of refraction. The polariser being then turned until the 

 ray in question disappears from the extraordinary spectrum its 

 position is noted. A known small rotation is then given to the plane 

 of polarisation of the incident light either by turning the polariser 

 through a known angle, or introducing into the path of the light a 

 cell containing a solution of sugar. This causes the reappearance of 

 the ray in question in the extraordinary spectrum, and the spar prism 

 is then moved, thus varying the angle of incidence until it again dis- 

 appears. The angle of incidence and the deviation of the same ray in 

 the ordinary spectrum being measured, we get a second pair of values 

 of and 0' under the condition that the ordinary ray only traverses 

 the crystal. Now if /3 be the angle which the optic axis makes with 

 the edge of the prism, X the angle between the face of incidence and 

 a plane through the optic axis and the edge of the prism, and 6 the 

 angle between the direction of vibration of the, incident light and this 

 same edge, it follows, either from the electro-magnetic theory of light 

 (Lorentz, " Schlomilch Zeitschrift," vol. 22 ; Fitzgerald, "Phil. Trans.," 

 vol. 171, 1880), or the theories of Neumann (" Abhand. Akad. Berlin," 

 1835), MacCullagh (" Trans. Roy. Irish Acad." 1839), and Kirchhoff 

 ("Abhand. Akad. Berlin," 1876), that if the ordinary ray only 

 traverses the crystal — 



cot #=tan cos (X + 0') sec (0—0'). 



We are thus able to obtain a series of theoretical values of 0. The 

 difference between two consecutive values should, if the theory be 

 true, give us the angle through which the plane of polarisation has 



