of Minerals in the Thin Section. 345 



horizontal circles, H 1 (microscope stage), H 2 and H 3 , each circle 

 graduated into degrees with verniers attached to JET, and H 2 ; 

 each of these circles is revolvable about a vertical axis; the 

 horizontal axes of revolution and equivalent vertical circles are 

 Yj and Y 2 also divided into degrees and Y x with vernier 

 attached. On the original stage described by Fedorow and 

 made by Fuess, the partial scales Y 2 are wanting and ha\»e been 

 attached by the writer. These scales were constructed in the 

 workshop of the Geophysical Laboratory, and have been found 

 of practical service in several methods, especially those involv- 

 ing the principal sections of the triaxial ellipsoid of any min- 

 eral (page 353). Each partial scale of Y 2 is accurately divided 

 and carefully adjusted to the instrument. When not in use, 

 tha scale segments of Y 2 can be inclined to a horizontal posi- 

 tion Y 2& and are then entirely out of the way. Measurements 

 given below will be referred to this modification of the Fedo- 

 row-Fuess universal stage. 



To increase the angle of vision of the field, two glass hemi- 

 spheres, A, and A 2 (in fig. 20 A 1 only appears, A 2 being hidden 

 by H 2 ), are usually employed ; between these the preparation 

 is placed, either cedar wood oil or glycerine being used to 

 stick the same together and to reduce the effects of total reflec- 

 tion. For general work with the universal stage, it is advisable 

 to follow the suggestion of Fedorow and use special circular 

 (2 cm diameter) object glasses on which to mount the prepara- 

 tions in place of the ordinary rectangular (26 x 46 mm ) thin sec- 

 tion object glasses. 



With the universal stage of this type, it is possible not only 

 to bring a crystal section in any given position, but also to 

 revolve that section about any axis ; in short, by its use one 

 has control over all possible directions and zones or axes of 

 revolution of a crystal. 



The Determination of the Crystal System of a Given Min- 

 eral by Means of the Universal Stage Method. — The fact that 

 the universal stage allows the observer to study the different 

 effects of a given mineral section on light waves transmitted 

 through it in different directions, enables him to determine at 

 once the crystal system to which the crystal belongs. This is 

 accomplished most readily by means of extinction angles along 

 certain directions, since the term extinction angle implies a 

 definite relation between a given crystal lographic and a given 

 optical direction in any mineral. These relations vary with 

 the crystal system of the mineral, and in fact are such definite 

 functions of the same that, as Brewster* was the first to show, 

 it is possible from extinction angles alone to determine defi- 

 nitely the crystal system of a given mineral. Briefly, an iso- 



* Brewster, D., Phil. Trans., 1814, 187-218; 1818, 199-272. 



