242 Mr. C. Spurge. On the Effect of Polish on the 



" On the Effect of Polish on the Reflexion of Light from the 

 Surface of Iceland Spar." By C. Spurge, B.A., St. 

 Catherine's College, Cambridge. Communicated by R. T. 

 Glazebrook, M.A., F.R.S. Received November 18,— Read 

 December 16, 1886. Revised March 3, 1887. 



I. Introduction. 



The optical effect of polishing the surface of a transparent body has 

 received the most complete investigation at the hands of Seebeck,* 

 and till very recentlyt Seebeck's were almost the only experiments 

 made on the subject. Seebeck's method consisted in observing with 

 a Nicol the light of a lamp reflected from the surface of the body. 

 By means of a divided circle, the angle of polarisation was measured, 

 and it was from an alteration in this angle that a change in the state 

 of the' surface was inferred. But it has been since shown by JaminJ 

 that, when plane polarised light is incident upon the surface of a 

 transparent body, the reflected light is in general not plane but to a 

 measureable degree elliptically polarised, and consequently there is no 

 angle of incidence at which the light can be completely quenched by a 

 Nicol. It follows that, as regards our present state of knowledge, 

 Seebeck's investigation is to some extent incomplete, and also that 

 there is some uncertainty in the determination of the angles of 

 polarisation, which may affect our conclusions as regards the state of 

 the surface, especially since the difference produced by polishing is 

 according to Seebeck not very large. Both Sir David Brewster § and 

 M. Jamin were of the opinion that Seebeck's experiments should be 

 repeated, and the latter promised to consider the effect of polish 

 later on but appears never to have done so. Mr. Glazebrook kindly 

 pointed out to me that the subject presented a suitable field for 

 research, and, at his instance, I undertook the present investigation. 



My object has been to attain greater accuracy than hitherto by 

 employing for an analyser a quarter undulation plate in addition to a 

 Nicol, so as to make the extinction of the reflected light very 

 complete. The angle of incidence of the polarised light falling on the 

 surface of the crystal was kept constant, in order to measure as 

 directly as possible the alteration produced by change of polish. Both 

 the azimuth of the major axis and the ratio of the axes of the 

 elliptically polarised light were calculated. These quantities furnish 



* ' Poggendorff, Annalen,' vol. 20, 1830, p. 27 : Vol. 21, 1831, p. 290. 



f Sir J. Conroy, 'Roy. Soc. Proc.,' Feb., 1886. 



t 1 Annales de Chimie,' vol. 29, 1850, p, 263„ 



§ « Edinb. Journ. Sci.,' vol. 5, 1831. 



