107 



consists of two systems of crossing twin-lamellae. One system 

 runs parallel to the outer edge of the section , the other is at 

 right angles to it. The whole section is doubly refracting. The 

 extinction -directions make angles of 60° or 30° with the 

 longitudinal direction of the twin-lamellae. As, now, the outer 

 edges of the section and, consequently, the twin-lamellae make 

 angles of 63° 26^ and 26° 34', respectively, with the rectangular 

 axes of symmetry a and è, it follows that the extinction-direc- 

 tion, e. g. in the fields I makes an angle of 3° 26' with the 

 direction a — a. The fields II have an equal angle of extinction 

 on the other side. The fields III become dark simultaneously 

 with I, and the fields IV simultaneously with II. If the section 

 is placed in such a position that the directions a — a and Ъ — Ъ 

 coincide with the principal planes of the niçois , the extinction 

 is nearly total and quite the same throughout the section. 

 If it is revolved about 3^/2° to the right, total extinction in 

 the fields I and HI ensues; if it is revolved as far to the 

 left, the fields И and IV become quite dark. It is, however, 

 only the vibration -directions that coincide in the fields I and 



III on the one side, and in the fields II and IV on the other. 

 The direction of the greatest velocity of light in the fields I is 

 in the fields III the direction of the least velocity, and the same 

 is the case with the fields II and IV with regard to each other. 

 If, therefore, the section is placed so that the directions a — a 

 and b — b make an angle of 45° with the principal section of 

 the niçois, and a sensitive gypsum plate (red of the first order) 

 is inserted between the section and the analyzer, the fields I 

 and II show the same interference colour and the fields III and 



IV the complementary colour. 



22. Natrolite. 



This zeolite occurs on Narsarsuk still more sparingly 

 than analcime. Only three or four pieces with small crusts 



