3G2 O. Andersen — Aventurine Feldspar. 



reflecting lamellae for measurement. As a rule, however, they 

 were too narrow and the whole set of parallel faces had to be 

 adjusted as one face without regard to the intervening twin- 

 ning faces. By selecting the broadest set this could be done 

 without difficulty and after the adjustment the signals belong- 

 ing to the adjusted faces could be easily distinguished from 

 signals belonging to the other twinning faces. 



The determination of the angle <f> of any set of lamellae that 

 reflected light through the adjusted cleavage face would now 



Fig. 7. 



Collimator 



consist in measuring the angle between the following two zone 

 axes, both lying in the vertically adjusted face: (1) The zone 

 axis of the two cleavage faces (a-axis). (2) The zone axis of the 

 section lines of the lamellae. Each zone axis was in turn ad- 

 justed parallel to the vertical axis of the goniometer and the 

 position for each read on the vertical circle. 



The angles p were determined by measuring the angles 

 i and r in the following way : The cleavage piece was adjusted 

 with one face parallel to the vertical circle as before, and the 

 section lines of the set of lamellae to be measured were set 

 parallel to the vertical axis of the goniometer. The vertical 

 cleavage face was then fixed in a position which gave a suitable 

 angle of incidence to the rays from the collimator and readings 

 were made on the horizontal circle with the telescope in the 

 following three positions (see fig. 7) : 



(1) T u position of reflections from lamellae. (2) T 2 , posi- 

 tion of direct reflection from the cleavage face. (3) T 3 , posi- 

 tion of direct signal from the collimator. How the angles i 



