170 ROYAL SOCIETY OF CANADA 



the case, an optic a:xis is seen to emerge nearly normally when viewed in 

 convergent light. The following are the optical characters observed : — 



Axial plane = C (001). Ac. bis. -L A (100). 

 Crystal-axis a = a, b = y, c = p. 

 Birefringence, negative, very strong. 

 Dispersion, p <, v, not very marked. 



The orientation of the indicatrix axes is shown in fig. 1. 



The angle at which the optic axes emerge from a cleavage prism 

 was determined for yellow (JVa) light with the crystal in air (giving 

 the angle ^.l/»'''), and also after immersion in two oils of different 

 refractive index (giving SM"^^), and in one case a measurement was 

 made using red light (lithium flame). The following results were 

 obtained : — 



^^Aa = 82" 40' — mean of 12 experiments. 



^il/oi7, 1 48 _ yyp 23' " " 10 " 



^il/uj^, 1.558 ^ ^go 35' _ " '^ 6 



OHInl, 1.48 ^r~a r>' 



'^ L% = /i° approx. 



In order to calculate the true internal axial angle, 2V, & knowledge 

 of /3 was necessary. The first attempts to determine the refractive 

 indices were made by immersing a cleavage plate in an oil whose 

 index was higher than that of the mineral, using the total reflection 

 method. As the examination in parallel light had indicated a fairly 

 high refractive index, oil of aniseed (anithol) was used for this purpose; 

 its index was found to be 1.558 by minimum deviation. Previously 

 only the oil of /x = 1.48 had been employed in making the axial angle 

 determinations, but as the same instrument (a Fuess axial angle 

 apparatus) was to be used for measuring the refractive index, these 

 determinations were repeated with the oil of higher index ; this, although 

 not necessary, affords a useful check, since the true angle, 3V, should 

 be identical as calculated from the three sets of determinations, in air 

 and the two oils (see page 172). All that was necessary was to immerse 

 the crystal in the rectangular trough filled with anithol, adjust and 

 measure the axial angle; then sub'stitute a semi-cylindrical trough and 

 take readings for the two positions of total reflection. Yellow light 

 was used, and total reflection took place from a cleavage ||M. 



With the crystal mounted vertically, in which case the light travels 

 in a direction at right angles to the c-axis, we might expect to obtain two 

 shadows for total reflection — one corresponding to the index /3 and due 

 to light vibrating verticall}'-, and the other, vibrating horizontally, giv- 



