A Treatise on Crystallography

Deighton, 1839 - 139 Seiten

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Seite 21 - holohedral forms " of any system are those which possess the highest degree of symmetry of which the system admits. " Hemihedral forms" are those which may be derived from a holohedral form by supposing half of the faces of the latter omitted according to a certain law.
Seite iv - ... crystallographic notation proposed by Professor Whewell in his paper on a general method of calculating the angles of crystals, and the laws according to which they are formed, published in the Transactions of the Royal Society of London, 1825; and Professor Neuman's method of indicating the positions of the faces of a crystal by the points in which radii, drawn perpendicular to the faces, meet the surface of a sphere. The expressions which have been thus obtained are remarkable for their symmetry...
Seite 112 - ... and therefore if, from a single * "A twin-crystal is composed of two crystals joined together in such a manner " that one would come into the position of the other by revolving through two right " angles round an axis which is perpendicular to a plane which either is, or may be, " a face of either crystal. The axis will be called the twin-axis, and the plane to " which it is perpendicular the twin-plane." Miller's Treatiie on Crystallography, p. 103. In the text the word
Seite 96 - PCA sin PC = sin PAC sin PA; cos PZ = sin PAB sin PA = sin PBA sin PB; and also from these it follows that— -f sin PBA = ~ sin PBO ; ( A -|- sin PAC = y sin PAB ; 0 ~ sin PCB = A sin PCA. AA { = 180° - CAB ; i, = 180° - ABC ; - 180
Seite 9 - When three or more faces of a crystal have their poles in the same great cirele, they are said to form a zone, and the great cirele is called a zone cirele.
Seite 2 - Through any point (> within the crystal, let planes be drawn parallel to each of its faces and cleavage planes. Let OX, OY, OZ be any three intersections of those planes, not all in one plane; and let any face or cleavage plane meet OX, OY, OZ in A, B, C. Then, if any other face or cleavage plane meet OX, OY, OZ in H, K, L, and if we consider HO, AC.
Seite 117 - The upper or lower sign is to be taken according as the surface is concave or convex towards the incident light.
Seite 25 - ... therefore, only affect alternate octants of a crystal. The pentagonal dodecahedron, and hemihexakisoctahedron with parallel faces, do not, as I have shown, § 281, properly belong to the hemihedral combinations ; and they cannot, as I shall now proceed to show, cause the crystallographer any perplexity. " Any number of holohedral forms may occur in combination with each other, and with any hemihedral forms with inclined faces, or with any hemihedral forms with parallel faces.
Seite 12 - L e** + 1 where the upper or lower sign is to be taken according as...

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