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



Then 



Px = 



2ir. 



-P'x}t, 



and for a line y, 



Py = 







Consequently, 



Py _ 



Px 



x * P-y- 



-P>'y 

 -p!x 



Now X decreases from the lines A to E, and varies by considerably 

 over 30 per cent. The variations in the difference p,—p.' are very 

 much less if any analogy holds with Iceland spar and quartz, for 

 which the variation is less than 4 per cent.* Both causes tend to in- 

 crease p as the wave-length diminishes. Thus, somewhere about D 

 the retardation is 90°, and will approximate to the value derived from 

 Table I for white light, as we approach E. 



This discussion shows us that while the steel plate affords a more 

 accurate means of determining p for light of given refrangibility, yet 

 p is to a large extent a purely instrumental constant, such that its 

 value has no effect on the character of the results, and, when we con- 

 sider that white light was used in the previous experiments, the value 

 of p determined by Tables I, III, would seem to be confirmed by the 

 steel plate observations. We therefore accept the results of Tables I 

 — IV as determining the alteration in the polarisation of the reflected 

 light. 



IX. Statement of Results. 



The results of Tables I — IV are brought together in Table VII for 

 the sake of future reference and comparison, so as to exhibit a synop- 

 tical view of the final result up to this stage of the work. 



Table VII. 





tan sr. 



I. 



1st Angle. 



2nd Angle. 



Before polishing . . 

 After polishing . . . 



-03345 

 0-02517 



108 -088° 

 107 '819 



105 6 29 

 105 5 37 



74 58 35 

 74 58 44 



Difference .... 



-0-00828 



-l& 8" 



-52 



+ 9 



X. Variation of Surface State of a Polished Crystal with the Time. 



It is a point of importance to determine if the state of the surface 

 of a polished crystal is so permanent that it does not alter with the 



* Budberg, 1 Poggendoi'ff, Annalen,' vol. 14, p. 45 ; Grlazebrook, ' Phil. Trans.,' 1879. 



