1895.] Mineralogy. 835 
Professor v. Federow has kindly informed the editor of these notes that 
he has designed a third and simpler type of stage, specially adapted to 
petrographical work, which will shortly be described. All these forms 
can be attached to any of the standard types of petrographical micro- 
scopes by screwing to the mechanical stage. They require, however, a 
special form of slide, which is circular, with a diameter of about 2 em., 
and, when in use, this is held in an ebonite holder with circular open- 
ing, in which the slide can naturally be given any desired orientation. 
Parallel polarized light is used with this stage, and the presence of 
an axis of the ellipsoid of elasticity in any section is indicated by first 
bringing the two principal directions of the section parallel to the two 
axes of the stage and then rotating the slide about each separately. If 
either of the principal directions is an axis of elasticity, the slide will 
evidently remain dark when rotated about the axis normal to it, 
whereas otherwise it will show interference colors. This affords the 
following scheme for determining the symmetry of a mineral from ex- 
amination of random sections in a rock slide: 
Isometric. Every section is isotropic. 
Hexagonal and Tetragonal. Every section has one axis of elasticity. 
Orthorhombic. Sections lying in the zones of the three crystallo- 
graphic axes contain an axis of elasticity. 
Monoclinic. Sections belonging to the zone of the axis of symmetry 
contain an axis of elasticity. 
Triclinic. Entire lack of such sections. 
Some of Federow’s applications of this instrument to the Pen of the 
feldspars will be referred to later. 
A somewhat different form of stage embodying the same idea, but 
adapted to the study of the ordinary form of slides, has been since de- 
vised by Klein and manufactured by Fuess for attachment to his 
instruments. Klein‘ has also designed a form of this stage (likewise 
manufactured by Fuess for his large stand) to be used with convergent 
as well as parallel polarized light, and this can be used to find the 
position of the optic axes and measure the — angle in crystals as 
well as in sections. 
Connection Between Atomic Weight of Contained Met- 
als and Morphological and Optical Properties of Crystals. 
—The relations found by Tutton to exist between the atomic weights 
*Groth, Physikal. Kryst., 3d ed., p. 749, figs. 688 and 689 (1895). 
‘Ibiden, p. 750, fig. 691. Cf. also Sitzungsber. d. Akad. d. Wiss., Berlin, 
1895, p. 91. 
