PLATE B. 
Figs. 1—10.— Modifications of Crystalline Planes, Combinations. Axes, and Optical Phenomena. 
1 . Curved planes of ike h ex ah is - o eta h e dr on. or forty-eight-faced solid, as they occur in the diamond. 
2. Striatum, or striping of the planes of the cube in the direction of two parallel edges, as is occasionally seen in pyrites. 
3. Drusy or crystalline planes of the octahedron. 
4. Drusy surface of the cube , as it sometimes occurs in fluor-spar and lead-glance. 
5. Vertical striation of the hexagonal prism , united above with the rhombohedral planes, as is often seen in tourmaline. 
6. Horizontally striated lateral planes of the hexagonal prism, as they occur not nnfrequently in smoky topaz and rock- 
crystal especially. The pyramidal terminal plane and the hemihedral truncated plane of the lateral angles are smooth. 
7. Combination of the cube P with the octahedron 0, or truncation of the cube at the edges. 
8. Combination of the cube with the rhombic dodecagon , the planes of the cube are marked P, those of the dodecahedron D, 
right truncation of the edges of the cube. 
9. Combination of the cube with the pentagonal dodecahedron, or oblique truncation of the edges of the cube. 
10, Truncation or sharpening of the angles of the cube, the planes of truncation are placed on the planes of the cube, 
combination of the cube with the leucitoid. 
Figs. 11—18.—Axes of Crystals. 
11. Cube with three equal axes cutting each other at right angles, which are indicated by dotted lines, and connect the 
central points of opposite planes. 
12. Axes of square prism ; the principal axis is greater than the two equal horizontal axes, all three are perpendicular to 
one another. 
13. Axes of the right rhombic prism ; the three axes are perpendicular to one another, and are of different lengths ; the 
principal axis connects the central points of the basal planes, the horizontal axes those of the lateral edges. 
14. Axes of the oblique rhombic prism ; all the three axes are unequal, the two secondary axes are perpendicular to one 
another, the principal axis is perpendicular to one of these, and oblique to the other. 
15. Axes of the oblique rhomboidal prism ; all the three axes are unlike, and are at oblique angles to one another. 
16. Axes of the rhombohedron ; the principal axis connects the terminal angles, the three horizontal axes connect the central 
points of the lateral edges, and lie in a plane, which is perpendicular to the principal axis. 
Figs. 17— -21. —Optical Phenomena of Crystals, 
17. Represents a bit of Iceland double refracting spar, which lies on a sheet of paper provided with two concentric rings ; 
both of the rings appear doubled in consequence of the double refraction of rays, and they appear darker where they cover each 
other. By rotation of the prism, the images are displaced in various ways, and they are coincident so soon as the eye is in the direc¬ 
tion of the principal axis (optical axis). 
18. Iridescent rings of colour, with a dark cross, as seen in double refracting spar, and in all uni-axial crystals when placed 
in polarised light, as for instance in tourmaline plates ; the plates are cut parallel to their axis, and lie crossed. The double- 
refracting spar is cut perpendicularly to the principal axis. 
19. Iridescent rings, with a white cross, as they appear in crystals with one optical axis when in tourmaline plates, if the 
plates lie parallel; all the colours here are complementary to the corresponding parts in fig. 18. 
20. Appearance of the colours presented by crystals with two optical axes (as salpetre) in polarised light, when the crystal 
is cut perpendicularly to the middle line (the line dividing into two equal parts the angle of the two optical axes), and laid 
between the crossed plates of tourmaline, provided that the plane of the optical axis is coincident with the plane of rotation of one 
of the tourmaline plates. 
21. System of rings of bi-axial crystals, when the plane of the two optical axis is at an angle of 45° with the plane of revo¬ 
lution of both tourmaline plates. 
