OF DENISON UNIVERSITY. 
135 
distinguished from the former, even in sections, by the regular contour and 
cleavage. 
2. The remainder of mineral species are anisotropous. Tetragonal and 
hexagonal minerals have different elasticity in directions parallel and perpendicular 
to vertical axis c. Rectangular or hexagonal longitudinal sections are extingui.shed 
perpendicularly. In convergent polarized light transverse sections exhibit a 
fixed axial cross, while in longitudinal sections no interference figures appear. 
Sections obique to the axis show lateral displacement of the optic axis. . The 
fixed axial cross appears to move- in the same direction as the stage when revolved. 
Hexagonal are distinguished from the tetragonal forms by the hexagonal and 
twelve-sided transverse sections. 
3. The remaining minerals are optically bi- axial, and in those sections which 
are isotropous a black band appears in convergent polarized light, which appears 
to move in a direction opposite to the stage. The axial point may lie within or 
out of the field (depending on the inclination of the section.) If the band 
(hyperbola) is bordered with red on the convex side and blue on the concave the 
dispersion of the axes—py>v, if the reverse p<y. 
Orthorhombic minerals have both optic axes in a plane parallel a pinacoid. 
In sections at right angles to the vertical axis extinction takes place parallel and 
perpendicular to the sides. In sections parallel to the vertical axis extinction is 
also perpendicular. No interference figure in the third pinacoid section in con- 
vergent polarized light. 
In Monoclinic minerals one axis of elasticity coincides with the orthodiagonal, 
the others in the symetry plane i-i or perpendicular to it. Sections perpendicular 
to the vertical axis are perpendicularly extinguished. Sections parallel to the 
vertical axis if in the zone O . i-F are extinguished perpendicularly, sections 
parallel to i-i^are obliquely extinguished. If the optical axial plane is parallel to 
i-i none of the pinacoid sections exhibits perpendicular displacement of a bisectrix, 
but the interference figure is displaced toward the vertical axis. 
In Triclinic minerals neither of the axes of elasticity coincides with the crys- 
tallographic axes. The optical axial plane is not perpendicular to pinacoid sur- 
faces. No pinacoid section is perpendicularly extinguished and none of these 
sections exhibits perpendicular displacement of the bisectrix. 
Explanation of Plate XI. 
Fig. I., Diagramatic section of Lithological Microscope as manufactured by 
Fuess of Berlin. [See text.] 
Fig. 2. Reflecting goniometer of Wollaston, for measuring angles of macro- 
scopic crystals. 
Fig. 3. Polysynthetic twins of plagioclase ; section parallel i-I. (From 
Hussak.) 
Fig. 4. Plagioclase twinned according to both the albite and pericline type ; 
section parallel i-I. (From Hussak.) 
Fig. 5. Diagram illustrating relation of the optical axes etc., in twins. 
(In A X = Normal.) It may be more clearly illustrated as follows : — A thin 
plate of gypsum is taken parallel the principal clinodiagonal, Out of this a 
