IN TRUE rEllSPECTIVE. 
the greatest facility, be subjected to such calculations as 
should be rendered necessary by some particular question ; 
but it will be always found more convenient to follow the 
graphic process developed above, in which the only thing 
required for attaining the greatest precision, is some accu- 
racy in employing rulers and compasses. 
Having succeeded in projecting the hexahedron, we find 
ourselves enabled to resolve a great many problems respect- 
ing crystallographic drawings, some of which may here be 
considered, in order to attach to them a few observations 
relative to certain advantages in executing the projection. 
Problem II. To draw a regular Octahedron. 
If the octahedron and the hexahedron are brought into a 
parallel position, the only one in which they are produced 
by nature in one and the same crystal, the pyramidal axes of 
one of these forms coincide with the pyramidal axes of the 
other, and consequently there will exist an octahedron, 
whose size is exactly such, that its solid angles touch the 
faces of the hexahedron in their centres. Hence, determine 
the centres of these faces C, C", C"^ C^^, and 
Fig. 8., and join them by straight lines, the result will be 
the octahedron. 
Problem III. To draw a given Isosceles Four-sided Py- 
ramid-, Jbr i7istance, P of Pyramidal Zircon. 
According to Mohs, a in this species is =z that 
is to say, the axis of the pyramid P is — the side 
* * Treatise on Mineralogy,* vol. ii. p. 368. The measurements upon 
which this quantity depends, were taken upon the small, but beautifully- 
formed, crystals which accompany the native platina, and agree exactly with 
the results obtained by Dr Wollaston, and Messrs Brooke and Phillips. 
