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



Table XV. — Second Series of Observations with a Natural Face. 





r'. 



r. 



R'. 



R. 





137 '020° 



42*589° 



134 -259° 



42 -682° 





136-935 



42 -804 



133 -974 



42 -539 





137*105 



43 -015 



133 -882 



42 -464 





136 -819 



43 -032 



133 -994 



42 -581 





137-046 



42-496 



134-302 



42-510 





136 -884 



42 -744 



134 -325 



42 -652 





Loo vQAJ 





loo /ol 



i k£i DD-i 





137 -216 



42-674 



134-091 



42-556 





136-900 



42 -971 



133 -921 



42-710 





136 -781 



42-767 



134 -317 



42-630 





136 -961 



42-791 



134-337 



42 -636 





137 -082 



43-030 



134 -285 



42 -515 



Means . . 



136 -974 



42-825 



134 -122 



42 -594 



Thus we find that I = 88*358°, 



and tan *r = O03368. 



We have now to compare these results with those recorded in 

 Table VII, taken some fifteen months previously. Originally we 

 had — 



tan "sr = 0-03345, 

 whereas the present value is — 



tan *r = 0*03368. 



The agreement is very close, and, as might be expected from a com- 

 parison of Table VII with the results of Table IX, the latter number 

 is somewhat larger than the former on account of the resetting of the 

 instrument. Again, the first series of observations show that the 

 effect of polishing was according to Table VII to change I from 

 108"088° to 107"819°, a decrease of 0'269°, whereas the second series of 

 experiments show that the result of repolishing has been according to 

 Tables XII and XV to change I from 88-358° to 87-712°, a decrease of 

 0'646°. This result is correct as regards sense, but not so satisfactory 

 as regards magnitude. It must, however, be remembered that the 

 breaking up of the base of the crystal would most probably have an 

 influence on the value of I. 



We are now in a position to sum up the results of this investiga- 

 tion. The process of polishing a natural face of a crystal of Iceland 

 spar with emery and rouge does most certainly alter the state of the 

 surface. This alteration is evinced by a change both in the ratio of 



