1886.] On the Reflexion of Light from Iceland Spar. 463 



IV. " 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, 

 1886. 



(Abstract.) 



The most complete experiments which have been hitherto instituted 

 to determine the optical effect of polishing the surface of a trans- 

 parent body are those of Seebeck, described in ' Poggendorff, Annalen,' 

 vol. 20, 1830, p. 27 ; vol. 21, 1831, p. 290. Seebeck's method was to 

 quench, as far as possible, the reflected light with a Nicol, and to 

 measure the angle of incidence. It was from a change in the angle 

 of incidence, i.e., the angle of polarisation, that an alteration of the 

 state of the surface was inferred. 



This method is open to the objection that the light is not completely 

 quenched, and, therefore, since the effect of polishing as observed by 

 Seebeck was not very large, our conclusions as regards the surface 

 state may be modified. Besides, the investigation is incomplete, for 

 the precise change produced in the reflected light by polishing is not 

 determined. 



The present paper, of which this is an abstract, is an account of 

 experiments made to determine with greater accuracy the effect of 

 polishing the surface of a crystal of Iceland spar, and also the exact 

 alteration produced in the reflected light, i.e., the change in the ratio 

 of the axes of the ellipse, and in the azimuth of the major axis of 

 the eliiptically polarised light. 



To effect this, an elliptic analyser, consisting of a Nicol and a 

 quarter undulation plate, was employed. If r, r be the mean readings 

 of moveable verniers attached to the Nicol in the two distinct 

 positions in which the light is extinguished, and R, R' similar 

 readings of fixed verniers which determine the azimuth of the 

 quarter plate, then tan w, the ratio of the axes, and I, the azimuth of 

 the major axis of the eliiptically polarised light, were calculated from 

 the formulas — 



cos 2*r = sin (r'-r)/ sin (R'-R) and I = (R + R')/2. 



Especial care was taken to secure fixity of position in all permanent 

 parts of the instrument, and in the setting of the face of the crystal, 

 since a small change was to be detected. 



A first series of experiments was made with light reflected from a 

 natural face, and with light reflected from the same face when 

 polished. The polishing was performed by myself, and precautions 



