Reviews— Nikitin's Methode Universelle de Fedoroff. 425 



away by the wind or the occasional rainfall. Therefore the general 

 denudation of the land is very powerful. The Efryptian monuments, 

 exposed during 4,000 years to the disintegrating and denuding 

 powers of the desert, offer beautiful examples of the different kinds 

 of dry disintegration, and many of them show very clearly also the 

 transporting effect of the wind. 



E,E!A7^IEA^^S. 



T. — W. W. NiKiTiN. La Methodf: Univeeselle de Eedoboff. 

 Traduction frau^aise par L. Duparc and Mile V. de Dervies. 

 Two volumes in one; pp. 516 and atlas. Geneve, Edition Atar ; 

 Paris, Beranger. 30 francs. 



THE universal or theodolite methods of investigating tlie geo- 

 metrical and optical properties of crystals were first fully described 

 by Professor Fedoroff in the year 1893. The problems connected 

 with the geometrical study are relatively simple, and the original 

 theodolite or two-circle goniometer scarcely admitted of any essential 

 improvement. On the other hand, the full determination of the more 

 important optical constants of a crystal fragment or of a mineral in 

 thin section obviously pi'esents difficulties of a far higher order, 

 which have only lieen successfully surmounted after the expenditure 

 of much time and ingenuity on the part of the creator of the method 

 and of his pupils, amongst whom the author of the present work 

 must always take the highest place. The employment of the 

 Fedoroff universal stage in its present perfected form cannot fail to 

 revolutionize the optical study of crystals, not merely by reason of the 

 fact that a mineral can be identified with great rapidity, but because 

 it is now possible to determine optical constants with such a high 

 degree of exactitude as to settle within narrow limits the position of 

 the mineral in its isomorphous gronp. This is especially true of the 

 felspars, the optical data of Avhich have been so extensively and 

 carefully studied. 



The work is a translation with many important additions of a 

 comprehensive guide to the use of the universal stage, which was 

 especially written by Professor Nikitin for the use of the students in 

 the St. Petersburg School of Mines. Its scope may be best indicated 

 by the following table of contents: — 



Tome i : Introductory chapter on the optical properties of crystals, 

 pp. 1-59. Graphical methods, especially those relating to the use of 

 the Fedoroff stereographic net, pp. 60-91. Description of the 

 universal stage and its adjustment on the polarizing microscope, 

 pp. 92-120. Discrimination of optically isotropic, uniaxial, and 

 biaxial crystals, ])p. 121-39. Determinations of the orientation and 

 optic axial angle of a biaxial crystal, pp. 140-89. Approximate 

 determination of the refractive index, 190-237. Recapitulation of 

 methods for a biaxial crystal, pp. 238-49. Uniaxial crystals, 

 pp. 250-69. 



Tome ii : Determination of the thickness of the section and of the 

 birefringence, pp. 275-443. Determination by extinction methods of 



