[graham] 



DAWSONITE 



171 



ing the index for light travelling through the crystal at right angles to 

 M (110), which would have a value soniewhere between a and y. 

 Only one shadow could be found, however; the angle between the two 

 positions of total reflection was 151° 13' (mean of 4) giving 75° 36^'' 

 as the angle of incidence (fig. 3), whence n = 1.558 sin 75° 36^' = 1.510. 



Examination of the reflected light by means of a Nicol prism 

 showed it to be vibrating horizontally, and the index obtained is there- 

 fore that for light travelling normal to (110) in the crystal; as will 

 appear later, it lies between a and /3. 



The crystal was then suspended with its length horizontal and an 

 M (cleavage) face in a vertical position. Total reflection from M 



Fig. 3. Total Reflection Method. 



in a suitable medium would give two dark bands corresponding to a 

 and y; but since the above experiment had not yielded readings for the 

 mean index, y5, it was not to be expected that the y ray would suffer 

 total reflection in the present case; for the index y is higher than 1,558, 

 as was found later. The line due to the total reflection of the a ray was 

 sharply defined, giving 140°20' (mean of 4) as the angle between the 

 two positions, which corresponds to an angle of incidence of 70°11'; 

 and a. calculated from this is found to be 1.4664. 



The light reflected from the plate was extinguished with the Nicol 

 at approximately 45 ° ; I have not calculated the theoretical directions 

 in which the vibrations would take place in this particular case, where 

 light, travelling parallel to /3 in the crystal, emerges, at the critical 

 angle, as two plane polarised beams, ftom a face of the rhombic prism; 

 but they would obviously be in some oblique position. 



