d 



Petrological Abstracts and Reviews 



ALBERT JOHANNSEN 



Berek, M. "Zur Messung der Doppelbrechung hauptsachlich 

 mit Hilfe des Polarisationsmikroskops," Centralhl. J. Min. 

 Geol. u. Pal., 1913, 388-96, 427-35, 464-70, 580-82. Figs. 7. 

 The new Berek compensator here described is somewhat similar in 

 pattern to the Nikitin quartz compensator. In the present case, how- 

 ever, the mineral plate is calcite. The author shows that the dispersion 

 of mineral sections nearly at right angles to the optic axis becomes prac- 

 tically zero, consequently any inactive, uniaxial mineral is suitable. 

 Since rather a thick section is required to give the higher orders of colors, 

 quartz cannot be used on account of its rotary polarization. The 

 construction of this instrument permits much more accurate measure- 

 ments than that of Nikitin. A calcite plate, o.i mm. in thickness, cut 

 at right angles to the optic axis, is attached to a rotatable axis in a 

 slider which is to be inserted in the slot in the microscope above the 

 objective. Since it is thus between the polarizing prisms, no cap nicol 

 is required. The amount of rotation of the plate is read from a graduated 

 drum and vernier to 2'. With the thickness of calcite used, it is possible 

 to read colors to the fourth order. If higher readings are desired, the 

 calcite plate may readily be removed and replaced by one that is thicker. 

 The sensitiveness of this compensator is shown to be equal to that of 

 Babinet for moderate double refraction, and greater for low colors. 



Berek, M. "tjber Zirkularpolarisation," Fortschr. d. Min., 

 Krist., u. Petr., IV (1914), 73-114, 221-62. Fig. i. 

 A full discussion of the phenomenon of rotary polarization. 



Berek, M. "Die astigmatischen Bildfehler der Polarisationspris- 

 men," Centralhl. f. Min. Geol. u. Pol., 1919, 218-24, 247-55, 

 275-84. Figs. 5. 



The insertion of the nicol prism between the objective and ocular 

 causes astigmatic and focal disturbances. The latter is usually com- 



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