CALCULATION OF REFKACTIVE INDEX. 183 



Dr. Cotton's method, however, affords a means of calcu- 

 lating the refractive index in any random direction. In 

 applying this to the plagioclase felspars the directions 

 selected were those paralled to (010) and (001). Such 

 sections are parallel to the two prominent cleavages and 

 may be readily obtained in practice. 



The data required for the construction of the stereo- 

 grams were obtained from Michel Levy's "Etude sur la 

 Determination des Felspaths," and the values of a, /3 and y 

 necessary for the solution of the equation 

 P m 2 n 2 1 



a 2 + /3 2 + f ~~ 



r~ 



from Idding's "Rock Minerals." They are here given in 

 tabular form. 



The positions of the elements a, f3 and y and of the optic 

 axes A and B as well as the crystallographic forms (001), 

 (010) and (100) are thus defined by the values of 4> and p. 

 The values of </> are measured in the plane ^of projection 

 (vide Fig, 1) and the values of p are the distances of the 

 elements from the centre of projection. This nomenclature 

 is the same as that conventionally adopted in crystallo- 

 graphic work. 



Values for r l and r 2 were obtained for both the (010) and 

 (001) sections of each felspar and graphs constructed. These 

 are given in Figs. 2 and 3 respectively. 



A series of liquids was then prepared witli refractive 

 indices equal to the mean of the values obtained for r : and 

 r 2 of the various felspars on the (010) and (001) faces 

 respectively. The liquids used were mixtures of clove 

 oil and momobromo-naphthalene and had the following 

 refractive indices. 



1. 1*535 3. 1*542 5. 1*558 7. 1*588 



2. 1*510 4. 1*550 6. 1*562 



These were then used to determine the refractive index 

 of small cleavage fragments by the Schroeder van der Kolk 

 method. 



