60 MERWIN : PROPERTIES OF ALKALI FELDSPARS 



optic axial angles of sanidines rich in albite are larger than those 

 of pure orthoclase. The presence of albite in orthoclase raises 

 the density, but apparently not so much as might be expected if 

 no volume change accompanied the mixing; the refractive index, 

 on the other hand, is generally higher than might be expected 

 under these conditions. As the percentage of albite increases the 

 length of the crystallographic axis a decreases. The angles in the 

 prism zone and in the zone of the basal and front pinacoids are 

 most affected by this change. Crystals in the same hand speci- 

 men often exhibit considerable variation in the value of these 

 particular angles, while other angles remain practically constant. 

 Such variations indicate that the different interfacial angles are 

 not equally sensitive to changes in the concentration of albite in 

 the orthoclase. The crystallographic axis c appears to be slightly 

 shorter in adularia than in sanidine. 



PETROLOGY. — A micrometer ocular with coordinate scale. Fred. 

 Eugene Wright. Geophysical Laboratory. 



This new eyepiece 1 (Fig. 1) consists essentially of a hollow 

 brass cylinder which fits into the microscope tube as an ordinary 

 eyepiece and serves as support for a positive Eamsden ocular in 

 the focal plane of which a finely divided 0.1 mm. coordinate scale 

 is placed. The eyepiece has been designed to serve three purposes 

 (1) that of an ordinal y micrometer eyepiece, (2) that of a coordi- 

 nate ocular for the geometrical analysis of rock thin sections either 

 by the Rosiwal method or by the less accurate method of percent- 

 age areas, and (3) for the measurement of the optic axial angle of 

 mineral plates in the thin section provided that at least one optic 

 axis appears in the field of view. In using this eyepiece for optic 

 axial angle measurements it is necessary first to ascertain the 

 equivalent angular value of its different coordinates for the par- 

 ticular lens system of the microscope with which it is to be used; 

 after this has been done, the observer has control over the whole 

 visible field and can measure the optic axial angle of sections which 



1 Made for the writer by R. Fuess Company, Steglitz, Berlin, Germany (cost 

 35 Marks) ; also by the Bausch and Lomb Optical Company, Rochester, N. Y. 

 The ruled coordinate scale was furnished in both cases by Swift and Son, London. 



