244 



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



each principal position. The position of the Nicol is determined by 

 the readings of the two moveable and that of the quarter wave plate 

 by the readings of the two fixed verniers. Thus each principal 

 position is determined by eight readings, and in the tables which 

 follow, each number is the mean of eight readings. 



Suppose that R, R' are the mean readings of the fixed, r, r the 

 mean readings of the moveable verniers, then the quantities, which it is 

 the object of the present investigation to determine, are tan -zzr, the 

 ratio of the axes of the ellipse, I, the azimuth of the major axis of the 

 ellipse, and these are given by the formulas — 



cos 2w = sin (r'—r)/ sin (R' — R), 



and I = |(R' + R). 



These equations determine absolutely, but I will be measured 

 from an arbitrary zero which will remain fixed so long as the quarter 

 plate is not unscrewed from its containing tube, but which will be 

 changed by a constant amount, if for any reason it is unscrewed and 

 rescrewed up. 



A subsidiary quantity is p, the retardation of the crystal plate, 

 which ma} 7 be determined by means of the equation, 



cos p = tan (r' — r)/tan (R' — R). 



The source of light employed was an Argand burner, the rays from 

 which were polarised by means of a Nicol before incidence. 



III. Adjustments. 



The tube. of the polariser was levelled and its axis placed in a direct 

 line with the centre of the flame. The height of a small brass table, on 

 which the crystal was placed was adjusted so that the reflected light 

 passed through the tube of the analyser. A number of preliminary 

 observations were made to determine the best angle of incidence, i.e., 

 the angle at which the extinction was most rapid. 



The best position of the analyser having been found, the centre of 

 the tube of the analyser was adjusted to the same height as the centre 

 of the tube of the polariser and the centre of the face of the crystal. 

 The tube of the analyser was so directed that a ray of - light from the 

 centre of the flame passing along the axis of the polariser was reflected 

 so as to enter at the centre of the tube of the analyser and leave at the 

 centre of the tube. 



As the present experiments were directed to discover a difference 

 which at the outset was recognised as possibly small, especial care 

 was taken to secure fixity of position in the parts of the instrument 

 and in the position of the face of the crystal of Iceland spar. 



