132 CARNEGIE INSTITUTION OF WASHINGTON. 



A device has been constructed in the Geophysical Laboratory to correct 

 this defect and has been found satisfactory and useful in practice. It 

 enables the observer to produce an aperture of any desired size in any part 

 of the field; he has thus control over the entire field and can eliminate any 

 incident rays which would otherwise disturb the kind of illumination desired. 

 He can, moreover, obtain rays of any desired obliquity of incidence and thus 

 increase or decrease the apparent relief of the surface under observation. 



(3) Are quantitative physico-chemical studies of rocks practicable? Arthur L. Day. 



Compte Rendu, XI Congres International Gcologiquc Stockholm, vol. ii, 

 pp. 965-967. 1910. 



An address before the Section on Mineralogy and Petrography of the 

 Eleventh International Congress of Geologists, in which an effort was made 

 to show by reference to recent laboratory studies of simple mineral relations, 

 using physico-chemical methods, that these methods must eventually find 

 application in the study of the very complicated mineral systems (the 

 igneous rocks) also. 



(4) Media of high refraction for refractive index determinations with the microscope; 



also a set of permanent standard media of lower refraction. H. E. Merwin. 

 J. Wash. Acad. Sci., 3, 35-40, 1913. 



A number of experimental studies have been carried on for the purpose of 

 extending the conditions under which determinations of refractive index by 

 means of the microscope can be made. Such determinations require immer- 

 sion media of standard refractive index. Various immersion liquids have 

 been in use for the determination of refractive indices over the interval 1.33 

 to 1.80; mixtures of amorphous sulphur and selenium have been found 

 useful over the range (for sodium light) 2.1 to 2.4. The immersion media 

 described have been devised to fill the gap 1.80 to 2.10, and to extend the 

 series beyond 2.4Na, or in special cases, particularly when a refractometer is 

 not at hand for standardizing the liquids, take the place of media hitherto 

 used. For the latter purpose solids have been found which may be mixed in 

 given proportions by weight to produce permanent standard media. 



(5) The index ellipsoid (optical indicatrix) in pctrographic microscope work. Fred. 



Eugene Wright. Am. Jour. Sci. (4), 35, 133-138. 1913. 



This paper emphasizes the importance of presenting the subject of 

 microscopical petrography consistently from the viewpoint of the index 

 ellipsoid as applied to wave-front normals. The various optical prop- 

 erties employed in practical petrographic microscope work can be best 

 described and explained systematically by means of the index ellipsoid. 

 The use of the so-called ''axes of elasticity," a, b, r, or X, Y, Z, in this con- 

 nection is confusing and only adds to the difficulties encountered by the 

 observer in mastering the subject. They should accordingly be abandoned 

 and the French usage of naming the principal axes of the index ellipsoid 

 a, /3, 7 (or Up, iim, rig) adopted. This applies in particular to the different 

 modes noAv in vogue for expressing extinction angles. For a given crystal 

 face an extinction angle is simply the angle between a definite crystallo- 

 graphic direction on the face and one of the axes, a' or 7', of its optic ellipse, 

 and this fact should be indicated in the expression for the extinction angle. 

 To introduce "axes of elasticity" (a',r',or X',Z') in this connection is not 

 only needless, but less direct, as it introduces entirely new conceptions which 

 experience has shown only tend to bewilder the student. 



