156 



b' 

 I'" 

 m' 

 m' 



READINGS 



116 3' 

 73 05' 

 54 26' 

 358 43' 



ANGLES 



b\m" =62 8'; bM" = 43 18' 

 b' A m"' = 62 8';bM'" = 43 29' 

 I'M"' =86 47' 

 m"' A m"=124 16' 

 m'" A m = 55 43' 



The signals yielded by the prism 1 are complex from striations 

 and therefore the angles vary considerably. 



From the above measurements the angles for the two prisms 

 are: for m, m" A m"' + m"'*m' = 111 25' -4- 2 = 55 42.5'; and 

 for 1, Ul' +1" A 1'" = 173 9 38' ^2 = 86 49'. As the prism 

 m has been selected as the unit prism, it will intersect the macro- 

 and brachy-axes at unit lengths, or these lengths will be in the 

 ratio of the unit on the b axis to the unit on the & axis. In order 

 to determine this ratio with sufficient accuracy for use in the draw- 

 ing of the crystal, lay off, Fig. 306, ob, equal to unity on the ma- 

 croaxis, say 5 cm., and draw oa, the brachyaxis, at 90, then draw om, 

 making the angle aom = 1/2 (55 42.5') = 27 51' ; from b draw ba 



perpendicular to om and 



Ore: : ~^b w here it cuts the a axis 



will be unit length from o, 

 as oa = unity on a, which 

 by measurement = .52 + , or 

 oa = 52/100 of ob. 



Having the units on the 

 axes a and b the parameters 

 and indices of 1 may now be 

 determined ; in the same 

 way from o draw ol, making 

 the angle bol = 1/2 (86 49') 

 = 43 25, and from b draw 

 bl at right angles to ol, and 

 where it cuts the a axis at 

 x is its intercept when it cuts the b axis at unit length, ox 

 is by measurement just twice oa ; the parameters of 1 will be, 

 therefore, 2 a : b: ooc and its indices (120). m = a:b:ooc, 

 (110). 



The faces b, b', since their normals bisect the angles of these two 

 prisms, is a pinacoid, and its indices and parameters may be writ- 

 ten at once as oo a: b: oo c, (010), the brachypinacoid. 



FIG. 306. 



