182 L. A. COTTON AND M. M. PEART. 



required values of h m x n x . Thus r x is determined and a 

 similar process will enable r 2 to be also evaluated. 



Where one or more of the direction cosines are closely 

 equal to unity the inaccuracies of the actual measurements 

 on the stereographic net may give rise to sensible inaccu- 

 racies in the results. In order to obviate this difficulty 

 we have recourse to the relation 



l\ + m\ + n\ = 1 



If the sum of l\ + ml + n\ as obtained from the observed 

 values be not equal to 1, the total should be made equal to 

 one by proportional change in the values of JJ, ml and nl, 

 and the corrected values so obtained should be substituted 

 in equation (6) in order to obtain the best value of r x . 



The method described above is simple in principle though 

 it may appear a little elaborate in detail. The author has 

 however, found this method quite simple in its application 

 and with the aid of a stereographic net the calculations 

 may be quickly executed. He has also found that the 

 principles have been easily grasped by his students who 

 have applied this method in their class work. In practice 

 it has proved very much more satisfactory than the purely 

 analytical method. 



Part II.— The application of Dr. Cotton's Method of 

 Calculating Refractive Index to the Felspars. 



By Miss Mary M. Peart. 



With variation in the composition of the plagioclase 

 felspars from albite to anorthite there is a gradual increase 

 in refractive index. The various plagioclases might there- 

 fore be discriminated by an accurate determination of the 

 refractive index in some given direction. Curves showing 

 the variation for the maximum and minimum values of the 

 refractive index (a and y) are given in various text books, 

 cf. "Idding's Rock Minerals," but in practice sections 

 containing the a and y are difficult to obtain. 



