300 



C. ANDERSON. 



From inspection one would be led to think that II is 

 twinned to III on r, forming a triplet with I similar to the 

 triplet VI, VII, VIII, but the angle between II and III is 

 only 56° 12'; the angles between the others agree well with 

 requirements. Of possible m-twins we have the following: 



Segments. Meas. b a b Gale, for m-twins. 



IV, XII 62° 36' "" 62° 46' 



VI, IX 117 12 117 14 



VII, X 117 16 117 14 



It should be remarked that here as in other cases all the 

 twinned pairs are not independent; if VI and VII and IX 

 and X respectively are twinned on r, and if VI is twinned 

 to IX on m, then VII and X must be twinned on m. 



There are also a number of individuals which approximate 

 to twin position : — 



Segments. Meas. b a b. 



n' l y 62° X q } 0alc ' for m " twin 62 ° 46 ' 



V, VII 58 28 



IV, VI 58 19 



X, V 58 48 f 



X, VI 59 55 J 



' Calc. for r-twin 57° 18' 



ix 1 ; v iii Si 29 ] 0alc - for *- twin 122 ° 42 ' 



All these diverging angles show the required variation 

 except the angle between II and III, but where so many 

 segments are concerned, the mutual relations are not so 

 clear as in simpler groups; one cannot see the wood for the 

 trees as it were. The text figure brings into prominence 

 the fact that the crystals group themselves round directions 

 at about 60° apart, namely 0° 25' (mean position of I, VII, IX), 

 60° 35' (mean position of VII, II, X, IV, XI), and 119° 35' 

 (mean position of III, V, VI, XII). We shall see later that 

 this orientation, which shows that the grouping is not 

 haphazard, is characteristic. 



