172 ROYAL SOCIETY OF CANADA 



The angle M:M for dawsonite being near 60°, a cleavage prism 

 should be well suited for a determination of its indices by the minimum 

 deviation method; but several which were tried failed to give sufficiently 

 luminous images, owing chiefly to the minute size of the faces. When 

 it was found that the two higher indices, /5 and y, could not easily be 

 determined by total reflection, other cleavage prisms were examined, 

 and finally one was found which gave good results. Using the cleavage 

 angle 65° 51', light travels along the acute bisectrix (a), and the in- 

 dices y5 and y are obtained. The prism actually used had an angle of 

 65° 57' and gave the following result for yellow light: — 



2 D = 96° 13' — light vibrating vertically = /i. 

 = 109° 31' — " " horizontally = )/. 



whence, /? - 1.5414 and ;/ = 1.5968. 



A second determination, made at a different time, but using the 

 same prism, gave /5 = 1.5424, y = 1.5965. 



Another prism was subsequently found, which, while not admitting 

 of the same degree of accuracy, fully confirmed the above figures ; it is 

 therefore considered that the determination may be relied upon to the 

 third place of decimals, giving/? = 1.542 and y = 1.596. 



Calculation of 2 V. 



The true internal optic axial angle, for yellow light, was calculated 

 as follows : — 



(1). From /3 and the angle designated above as 2M"^^^-^^^ i.e , the 

 apparent angle, where the optic axes emerge through faces of a prism 

 immersed in oil of /^ = 1.558. See fig. 4. . 7»'^^'-.. 



;gilf=76°35' 

 M : M = 65°5r 



10°44' 

 .-. i= 5° 22'. 



«-^ = rli-«^^^^22'. 



and, r = 5° 253^'. 



Whence, ^F = 2 (5° 25^') -f 65°51' 

 = 76° 42'. 



(2). From /3 and 2M for oil of /^ = 1.48 (77° 23'). 

 The oil in this case has a lower index than the crystal, so that 

 S V will be less than 2M', by a calculation similar to the above it is 

 found to be 76° 56'. 



(3). From /? and the angle 2M for air (82° 40'). 



