GEOPHYSICAL LABORATORY. 95 



The calorimeter resembles in plan a figure 8 with a propeller in one side. 

 Dead spaces are completely avoided. 



Calibrations by the electrical method are still in progress. Seven pre- 

 liminary results with inferior sensitiveness have already shown an average 

 variation of only 2 parts in 10,000. 



(6) Die Kalkkieselreihe der Minerale. Arthur L. Day, E. T. Allen, E. S. Shepherd, 



Walter P. White, Fred Eugene Wright. Min. Petr. Mitt. (Vienna), 26, pp. 

 169-232, 1907. 



A translation into German, with some additions, of the papers "On 

 wollastonite and pseudo-wollastonite, polymorphic forms of calcium meta- 

 silicate" and "The lime silica series of minerals," reviewed in the last annual 

 report. 



(7) Measurement of the optic axial angle of minerals in the thin section. Fred Eugene 



Wright. Amer. Journ. Sci., (4) 24, pp. 317-369, 1907. 



The questions involved in the general problems of rock formation require 

 for their solution definite data on the conditions of formation and the prop- 

 erties of rock-making minerals. To furnish accurate and reliable data of 

 this sort it has been found necessary to determine the relative accuracy of the 

 methods available for the determination of the different constants, both 

 physical and chemical, of these minerals. In this paper the various methods 

 for the measurement of the optic axial angle of minerals under the microscope 

 are considered in detail and their relative accuracy tested under different 

 conditions. The optic axial angle is one of the most characteristic con- 

 stants of biaxial minerals, and is used frequently in practical microscopic 

 mineral diagnosis. 



In convergent polarized light, methods for the measurement of the optic 

 axial angle are available for all sections in which at least one optic axis 

 appears within the field of vision. Of these, the method requiring the use of 

 the Becke drawing-table is of general application and furnishes results of a 

 fair degree of accuracy, the usual probable errors being about ± 1° if both 

 optic axes are visible, and ± 5° if only one optic axis is visible. More 

 accurate and som.ewhat simpler in manipulation and of the same general 

 application is the method involving the use of a new double-screw microm- 

 eter ocular, described for the first time in this paper. This ocular, combined 

 with the method of projection of Professor Wulff, is a general extension of 

 the Mallard method, and, like the Becke method, utilizes the rule of Biot & 

 Fresnel defining the planes of vibration for any particular direction of wave 

 propagation. With this ocular the probable errors of determination should 

 not exceed 1° if both optic axes are visible, or 3° if only one optic axis 

 appears in the field. The Mallard method and formula, which underlie these 

 methods, have been tested, and the agreement of formula with fact for the 

 special objective used found to be remarkable. 



In parallel polarized light the methods involving the Fedorow-Fuess uni- 

 versal stage furnish satisfactory results, providing the position of one optic 

 axis can be determined directly. In case both optic axes are outside the 

 field of vision, the values obtained are usually unsatisfactory and inaccurate. 

 If both optic axes appear within the field of vision, the error of determina- 

 tion by means of the universal stage should not exceed 1° , and if only one 

 of the optic axes is visible the accuracy decreases to ±5°. 



