Petrographic Microscope Work. 77 



field may prove useful. The ordinary refractive index, <u, of 

 calcite is 1*658 ; that of dolomite, 1*682. Grains of calcite 

 immersed in a-monobrom-naphthalene (&> = 1*658) show dis- 

 tinct color fringes ; such color fringes do not appear along the 

 margins of dolomite grains. To apply the method in the field 

 two object glasses and a small bottle of a-monobrom-naphtha- 

 lene* are necessary. The material to be tested is finely pow- 

 dered with the hammer and a small portion immersed in a drop 

 of the a-monobrom-naphthalene on the object glass. The drop 

 and immersed powder are then covered with the second object 

 glass and the whole tilted and examined through a pocket lens 

 pointed toward the sky. Oblique illumination is procured by 

 placing the finger in front of the object glass (between the glass 

 and the sky). The edge of the finger then appears out of focus 

 and indistinct ; grains of calcite within this semi-dark zone of 

 indistinct focus show the characteristic blue and orange mar- 

 ginal colors, while grains of dolomite appear simply white and 

 dark. Here and there a suggestion of color is to be observed 

 even with the dolomite, but with a little practice the eye be- 

 comes accustomed to these differences and the two substances 

 can be readily distinguished by this method. The writer has 

 had occasion to use the method repeatedly during the past 

 decade. 



The phenomena of convergent polarized light observed in 

 parallel polarized light. 



By the use of oblique illumination practically all of the phe- 

 nomena which are obtained in convergent polarized light can 

 be obtained without difficulty. The importance of oblique 

 illumination in this respect has been emphasized by Schroeder 

 van der Kolk,f v. Fedorow,:): and recently by Schneiderhohn.§ 

 Schroeder van der Kolk obtained oblique illumination (1) by 

 use of a stop above the condenser (method 4 above) ; (2) by 

 casting a shadow with his finger placed between the reflector 

 and the source of light (modification of method 3 above) ; (3) 

 by tilting the plate mounted on the top surface of a glass hem- 

 isphere. Fedorow used the universal stage for the same pur- 

 pose. Schneiderhohn recommends a stop (Schieberblende) in 

 the eye circle of the ocular. From the discussion in the first 

 section of this paper it is evident that the most effective 

 method for obtaining oblique illumination is by methods 2 and 

 3 above, unless accessory apparatus, as a glass hemisphere or a 

 universal stage, is to be used. 



* Readily obtainable from dealers in chemical supplies. 

 fZeitschr. Wiss. Mikros., viii, 456-458, 1892 ; xii, 188-192, 1895. 

 tZeitschr. Kryst., xxiv, 603-605, 1895. 

 glbid., 1, 281-241, 1912, 



