28 CRYSTALLOGRAPHY. 
form of the regular octahedron; 55, an octahedron lengthened 
horizontally ; 56, one shortened parallel to one of the pairs 
of faces; 57, one lengthened parallel to another pair, the 
ultimate result of which obliterates two of the faces, and 
places an acute solid angle in place of each. The solid is 
then six-sided, and has rhombic faces whose plane angles are 
120° and 60°. ‘The following figures illustrate corresponding 
changes in the dodecahedron (fig. 58). In fig. 59 the dodeca- 
58, 59. 


hedron is lengthened vertically, making a square prism with four- 
sided pyramidal terminations. In 60, itis shortened vertically. 
In 61 the dodecahedron is lengthened obliquely in the direction 
of an octahedral axis, and in 62 it is shortened in the same 
direction, making six-sided prisms with trihedral terminations. 
So again in the trapezohedron there are equally deceptive 
forms arising from elongations and shortenings in the same two 
ilirections. 
These distortions change the relative sizes of planes, but not 
the values of angles. In crystals of the several forms repre- 
sented in figs. 54 to 57, the inclinations are the same as in the 
regular octahedron. ‘There is the same constancy of angle in 
other distorted crystals. 
