PETROLOGICAL ABSTRACTS AND REVIEWS 727 



for the microscopic examination of fine-grained and artificial prepara- 

 tions are here considered with special reference to their degree of 

 accuracy and range of general application. 



In the introduction, the petrographic microscope and its several 

 mechanical and optical parts are described. This is followed by chapters 

 dealing with the methods for determining refractive indices, birefrin- 

 gence, and extinction and optic angles. There are also reproduced some 

 extremely useful charts, such as Wulff's stereographic net, Hilton's 

 gnomonic projection net, Fedorow's refractive index diagram, and 

 various others. 



The book is an extremely valuable contribution to the literature of 

 petrography, and is especially important for the author's comparative 

 study of the value of different methods. It may be studied with profit 

 by all advanced students. 



A.J. 



Wright, Fred. Eugene. "The Index Ellipsoid (Optical Indi- 

 catrix) in Petrographic Microscopic Work," Amer. Jour. Set., 



XXXV (1913), 133-38. 



The writer suggests abandoning the "elasticity ellipsoid" and the 

 symbols for the "axes of elasticity" in the explanation of the phenomena 

 of light in crystals. He would use instead only the indicatrix and the 

 symbols for the refractive indices, regarding the use of other symbols as 

 bewildering to the student. The reviewer's experience has been that 

 students can grasp much more readily the idea of an ease (or difficulty) 

 of vibration in a certain direction in a crystal, and a corresponding rate 

 of movement at right angles to it, than they can the inverse relation of 

 the refractive indices. The reviewer long ago abandoned the terms 

 "axes of elasticity" and substituted for them "ease of vibration axes." 



A.J. 



Wright, Fred. Eugene. "Oblique Illumination in Petrographic 

 Microscope Work," Amer. Jour. Sci., XXXV (1913), 63-82. 



A study of the cause and effect of oblique illumination in the study 

 of mineral plates. A sliding diaphragm in the focal plane of the con- 

 denser is considered by the author to be the best method for producing 

 this illumination. He points out that in the measurement of extinction 

 angles, central illumination with parallel plane-polarized light is essential. 



A. J. 



