■iiiC) Kozu find Washington— Augite from Sbromboli. 



From the close agreement of the results obtained by the 

 two differenl methods, it follows that the relative values 

 of the three principal refractive indices may be accepted 

 as almost correct, so far as these methods are concerned. 



The birefringence (7— a) was tested by another method, 

 using Babinet's compensator. In order to eliminate the 

 difficulty of determining accurately the thickness of the 

 mineral plate, a crystal of the Stromboli augite and one 

 of a diopside whose indices 11 were carefully determined 

 by the writer (Kozu), were affixed to a glass plate with 

 the side pinacoids (010) parallel to it, and both crystals 

 ground down to equal thickness and with their ground 

 faces in the same plane. 



The section thus made was used for the determination 

 of the retardation of light passing through the crystal 

 plates between crossed nicols. The divisions read by 

 the compensator were 1250 ± 10 for the Stromboli augite 

 and 1256 ± 10 for the Sano diopside. Hence, the 

 retardations produced by the two mineral plates are 



1250 X k = 1273-8 for Stromboli augite, 

 1256 X k = 1279-9 for Sano diopside, 



where * is the constant of the compensator, 1-0190 for the 

 wave length 589 w. If we take the value of (7— a ) = 

 0-0274 for the diopside as the standard, the corresponding 

 value for the augite is approximately 0-027. Though the 

 result obtained by this method is not accurate enough 

 for the discussion of the birefringence in detail, it is suf- 

 ficiently so to show that the value of (7— a) of the Strom- 

 boli augite is higher than those of augites which have 

 been examined by many mineralogists. 



The extinction angle (c A c ) on (010) for sodium light is 



11 The refractive indices of two crystals of this diopside, from' Sano, 

 Province Kai, Japan, were determined. They are 



Ext. angle 

 a y y—a 2V (obs) (c * C) on (010) 



1-6732 1-6795 1-7006 00274 

 1-6729 1-6793 1-7005 0-0276 58° 32-5' 39° 



These crystals commonly occur as large, untwinned phenocrysts, reaching 

 3 cm. in length, in a dike rock. The mineral is quite fresh, the fractured 

 surface showing a dark green color, and a thin section being almost color- 

 less. In some crystals the marginal part is brown and the center green. 

 The determinations were made on the green crystals. The mineral has been 

 considered to be common augite by Prof. K. Jimbo and others, but without 

 giving any optical data. In its lower refractive indices, higher bire- 

 fringences, and smaller angles of the optic axes and of the extinction, 

 it resembles much more the diopside from Ala and other localities. 



