360 



0. Andersen- — Aventurine Feldspar, 



the lamella and the two faces of the feldspar lie in the same 

 zone. Rays in the plane perpendicular to the a-axis of the 

 feldspar will, therefore, remain in this plane throughout and 

 the calculations of the relations between the different angles 

 are simple. We find easily the following formulae : 



P = 



X+i' - r' 



., sin i . , sin r 



sin % = ; sinr = 



n n 



i= X — ij 

 The meanings of the different symbols are indicated in fig. 5. 



Fig. 5. 



These formulae enable us to calculate the angle p of any set 

 of lamellae lying in the zone of two faces when the angle \ 

 between the faces is known and the angles *(or *,) and r of any 

 light ray can be measured. 



In the preceding discussion we have, in general, tacitly 

 assumed that the light was homogeneous. In fig. 6 the 

 incident ray a p is supposed to consist of white light. After 



