258 



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



Taking the mean values of /, r, R', R from Table IX, the elliptic 

 analyser gives 



sin ( r'-r ) sin 93'340 n qqq^qi 

 cos 2w = -t—~ — — - = ryi oqa = 0-998591, 

 sm (R — R) sin 91 380 



whence w = 1° 31*2', and we have for the values of the quantities 

 which fully determine the nature of the reflected light 



tan nr = 0-02625, and I = 87-654°. 



The previous experiments, the results of which are recorded in 

 Table VII, give 



tan <* = 0-02517, and I = 107-819°. 



The reason of the difference of the values of I is the change of the 

 index error. In order to compare the values of I found in a subse- 

 quent part of the paper with those already obtained, we must there- 

 fore subtract 20" 165°. The changes in tan w may be ascribed to two 

 causes — error in resetting the instrument so that it was not exactly in 

 its former position, and error of experiment. It should be noted that 

 the difference produced by the reversal of the instrument is not at all 

 comparable with the difference produced by polishing. 



In the case of the readings taken with the simple analyser, the 

 mean reading will correspond to the azimuth of the major axis of the 

 ellipse, determined by the elliptic analyser. Thus from Table 2£ 



I' = 266° 5-3'. 



XIII. Determination of the Effect of Botation of the Crystal Mace in 



its own Plane. 



It was suggested that the setting of the crystal was such as to 

 allow a rotation of the face in its own plane, which might possibly be 

 the cause of the differences hitherto observed. A wedge of 4° 27' 

 angle was constructed, and upon this the crystal was placed. So 

 delicate was the mode of setting by screens that, even with the labour 

 of hours, I was unable to turn the crystal into such a position that 

 the reflected ray emerged through the pinholes. I therefore took a 

 thin sheet of paper and gently wore its surface away so. as to obtain 

 a fine wedge which was drawn under a corner of the crystal. By this 

 means the reflected ray with some trouble was rendered visible. The 

 observations are divided in the following table into se^s of two or 

 three. Each set was taken after the crystal had been taken off the 

 wedge and replaced upon it in position by means of the screens. By 

 this means the accuracy of setting can be judged. 



