( 375 ) 



sin Vlij V 



•^''" ->imax = - — 



fV;.S' ,(■ ('Ot iV 



we get 



cot ((,nr,.r = 



ÜIH V Slti X \ / 1 — 



\^ ao8 X cot X 

 sin Vtg V 

 cos X cot X 

 sin^ T' -|- cos^ V cos^ x 



.sm" V 8171 X \/cOS^ X COt^ X — 'Sl7l^ ^^9^ ^^ 



sin V tg V {sin^ V -\- cos^ V cos^ x) 



cos^ X sin^ V 



sin X cos V\ y ~ 



sin^ X cos^ V 



si?i'^ V -\- cox"^ V cos"^ X 

 y^cos* X cos^ V — si7i'^ X sin* V 



(Ö) 



sin'^ V -\- coft' V cos^ x 

 Wlien ,/' =1 O, is 



cot Umax ~=^ ^ '"OS F. 



From whicli for olivine follows the value : 



= /A/f./(rb)c-o.s43°30' 

 = (±)35°57'22". 



In the following tigiire these results are graphically represented. 

 The black lines connect tlie poles of planes with equal positive, the 

 lines in black and white those of planes with equal negative extinction. 

 Herein the angles have been considered positive from the acute bi- 

 sectrix in the direction of the hands of the clock ; negative in the 

 opposite direction. 



The curves MM' and NN' , going through the optic axes, connect 

 the poles of the planes with the greatest (positive and negative) extinction 

 and with the same inclination with regard to the acute bisectrix. 

 The point in wdiich the curves mentioned intersect an isogyre, has 

 on that isogyre the greatest angulardi stance from (A For the rest very 

 little need be added to what is to be read from the figure. It shows 

 clearly that an extinction with regard to the acute bisectrix, which 

 deviates little from 0^, is confined to the immediate neighbourhood 

 of the principal planes of symmetry. 



