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



V. Determination of the Inclination of the Natural Face to the same 



Face Polished. 



The crystal was taken down from its position on the brass table, 

 and, in order to ensure that the face from which the light was 

 reflected was ground parallel to itself, the inclinations of the face to 

 two other faces which were left untouched, were obtained before and 

 after polishing. The measurements were made in the usual way with 

 the spectrometer. It was found that a fair image of the slit could be 

 obtained by reflexion at each of the faces. Three times the crystal 

 was completely dismounted and measurements of each angle taken. 





Before polishing. 



After polishing. 





First angle. 



Second angle. 



First angle. 



Second angle. 





105 5 56 

 105 6 15 

 105 7 15 



74 58 37 

 74 59 

 74 58 7 



105 6 37 

 105 5 7 

 105 5 7 



74 59 , 

 74 58 22 

 74 58 50 



Means . . 



105 6 29 



74 58 35 



105 5 37 



74 58 44 



The angles are almost unaltered and the differences are within the 

 limits of experimental error. Lines were drawn round the sides of 

 the crystal parallel to the edges, and after polishing remained still 

 parallel, which was an additional confirmation. The conclusion at 

 which we arrive is that the polished face was parallel to the natural 

 face. 



VI. Method of Polishing the Crystal. 



The natural face which had been the subject of the experiments 

 recorded in Tables I and II was polished. The polishing was per- 

 formed by myself to ensure an exact knowledge of the treatment it 

 received. 



I am indebted to Professor Threlfall for the use of the apparatus 

 and materials. As previous experimenters seem to have experienced 

 a difficulty in obtaining a surface polished in the same manner, it may 

 be well to state the exact mode of polishing the surface. 



That the crystal might be polished under the same conditions of 

 pressure, a rectangular block of lead was cemented to the crystal. In 

 polishing especial care was taken not to press on the crystal down- 

 wards, but to exercise only lateral pressure. The crystal was first 

 polished with emery on a plate of glass which had been rendered 

 plane by grinding on a slate. The emery was prepared as follows: 



