Reflexion of Light from Iceland Spar. 



263 



Again, Table VII shows that the effect of the first polishing was to 

 diminish I from 108-088° to 107-819°, a diminution of 0-269°; but the 

 effect of the second polishing has been to increase I from 87" 654° to 

 87*712°, an increase of 0*058°. Thus the effect of the polishings has 

 not been cumulative so as to cause the original deviation of polishing 

 to increase still further on second polishing. On the contrary, I has 

 approached by a small quantity the value it would have for a natural 

 fa.ce. The inference may be drawn that the first polishing produced 

 all the effect that polishing can bring about, and that the second 

 polishing differs from the first by a quantity which there is some 

 ground for regarding as an error of experiment. But if this is so, it 

 may be thought that the values of tan -sr, the second independent test 

 of a change in the reflected light, should also alter in the same manner 

 and sense as those of I have done. 



Now, Table YII shows that the effect of polishing is to diminish 

 tan^r from 0*03345 to 0*02517, a diminution of 0*00828. Table XIII 

 and the calculation following Table IX show that the effect of re- 

 polishing is to cause tan-srto increase from 0*02655 to 0*02723, an 

 Increase of 0*00068. Thus tan-sr changes sympathetically with I. 



In Table XIV are contained the results of the observations made 

 with the simple analyser, and. we have first to investigate the limits 

 of accuracy. In regard to single observations there can be no doubt 

 of the correctness of Sir John Conroy's observation that " in order 

 to obtain accurate results with observations of this kind, it is neces- 

 sary to make a large number of observations, an4 to take their mean."* 

 Thus, in Table XIV the means of tens of readings are recorded, and 

 for purposes of comparison the means of fifties are tabulated. 



Table XIV. — Observations with a Repolisbed Face (Simple 

 Analyser). 



Means Means 

 of ten of ten 



readings Means readings 



Means 

 of 50. 



Means 

 of 100. 



No. 1, 



of 1st of 50. of 2nd 



vernier. vernier. 



355° 52 -2' 176° 1 *2' 



355 50-3 175 59*1 



355 57-7 176 5:6 



355 49-9 175 59 -.3 



355 51-6 176 0-7 



355° 52-3' 



176° 1*2' 265° 56* 8' 



No. 2, 



355 59-5 176 6'1 



355 53-6 176 1*6 



355 58-3 176 7*8 



355 54-9 176 4 '6 



355 50-5 175 59*6 



355 55-3 



176 3 -9 



265 59*6 



* ' Eoy. Soc. Proc.,' Feb., 1886. 



