170 CARNEGIE INSTITUTION OF AVASHINGTON. 



(26) Die thermale Entwasserung: des Stilbit und Thauinasit; die Hydrate von Magnesium- 



und Kupfersulfat. H. E. Merwiii. Z. Kryst., 55, 113-114 (1915). 



A German translation of "The thermal dehydration of stilbite, thaumasite, 

 and the hydrates of magnesium sulphate and of copper sulphate" (J. Wash. 

 Acad. Sci., 4, 494-496, 1914). Reviewed in Year Book No. 13, p. 148. 



(27) Die Messung der Brechungsexponenten mit dem petrographischen Mikroskop. Fred. 



E. Wright. Z. Krj'st. (in press). 



The petrographic microscope may well serve as an instrument for the 

 measurement of refractive indices either of small drops of liquid or of minute 

 crystal grains. In this article the more important of the various methods 

 which have been suggested for this purpose are described briefly and with 

 special reference to accuracy and ease of manipulation. The methods for 

 measuring the refractive index of a liquid are based on one of the following 

 factors: (a) The change in position of focus of the lens system on the intro- 

 duction of a drop of liquid as an integral part of the lens system (methods of 

 Piltschikow, Clay, Smith, Decombe, de Souza Brandao, and several new 

 methods). These methods are in general accurate only to the second decimal 

 place. Their weak feature is the difficulty of sharp focusing; this results 

 chiefly from the disturbance of the corrections of the aberrations of the lens 

 system on the introduction of the liquid as one of its components. (6) Direct 

 comparison of the refractive index of one of a series of plates of crystals or of 

 glasses of known refractive index with that of the liquid. (Methods of Michel- 

 L4vy and of de Souza Brandao.) The chief difficulty in these methods is to 

 obtain a sufficiently large series of crystal or glass plates, so that the intervals 

 in refractive index are small, (c) Methods of total reflection. In this case 

 the microscope serves as an instrument for measuring the inclination of trans- 

 mitted light-waves, as in optic axial angle measurements. Five new methods 

 based on this principle are described, of which the simplest and most conven- 

 ient furnishes, under favorable conditions, refractive index values accurate 

 to =1= 0.001 . The method of Wallerant requires the use of a special total refrac- 

 tometer attachment; although more accurate than any of the other methods, 

 it has not come into general use because of the expense and trouble involved 

 in making the measurements, (d) The method of Fery is also based on the 

 phenomenon of total reflection, and is identical in principle with older methods 

 in which the width of the circle or band of total reflection on either an immersed 

 sphere or hemisphere of glass or a gas bubble serves as a measure of the rela- 

 tive refractive index of the adjacent liquid. This method is in general accurate 

 to the second decimal place. 



In the exact measurement of refractive indices of minute crystal particles 

 by the immersion method, it is essential: (1) that correctly oriented sections 

 be selected which are normal to at least one principal optic section; (2) that 

 in case oblique illumination be used, only those pencils of light be employed 

 whose direction of propagation is included in the plane normal to the principal 

 optical section; in other words, the metal sliding strip should be so inserted 

 that its front edge is parallel to a principal section; (3) that with central 

 illumination special weight be given to the effects along those edges of the 

 grain which trend approximately parallel to the principal section. If these 

 conditions be disregarded the value obtained for the maximum refractive 

 index will be too low, the value for the minimum refractive index too high, 

 while that of any intermediate refractive index, as (3, may be too high or too 

 low. Failure to recognize these factors may lead, especially in the case of 

 strongly birefracting crystal particles, to refractive index determinations 

 which are appreciably in error. 



