Simpler Methods in Crystallogra'phy. 427 



a few examples to be used as standards may well be given 

 here. Forms belonging to the Cubic System will be taken 

 first. The axial crosses should be drawn on rather thin, 

 good cardboard, and fine pin-holes should be pricked through 

 the chief points in the projection. In using the standard 

 cross, it is laid over the paper upon which the other drawing 

 is to be made, kept in position by weights, and then the 

 points are pricked through with a fine needle, mounted in 

 some kind of holder. The standard is then removed, and a 

 fine ring is pencilled round each of the punctures as lightly 

 as possible. Then these are connected by straight lines. I 

 prefer a hard pencil for this work, which should have a 

 chisel-edge instead of a point, and should be kept fine by 

 rubbing it on a piece of fine glass-paper, and then on a 

 piece of brown paper. I usually prefer to pencil in first the 

 unit octohedron, which is easily done. Next the pinacoids 

 are drawn, so as to project the outline of the cube. This 

 forms a good test of the draughtsman's skill, and the work 

 therefore needs to be as carefully done as possible. When 

 the outline of the cube is quite correct, it may advantageously 

 be carefully inked in with a ruling pen, an undercut straight- 

 edge, and Indian ink. If an ordinary straight-edge is used, 

 it is well to fasten two or three strips of cardboard on the 

 under side, so as to prevent the ink running from the pen to 

 the drawing in larger quantities than is desired. Next ink 

 in the cubic axes in some distinctive colour. This done, 

 divide each edge of the cube into halves, and join the middle 

 points of each pair opposite the centre by lines. These, also, 

 may be advantageously inked in of a different colour from 

 the rest. Then join the opposite solid angles of the cube 

 by lines through the centre, and colour these some distinctive 

 tint also. Call the cubic axes the Tetrad Axes, because a 

 revolution around each of them brings similar faces four 

 times into the same position in one revolution. Call the 

 next the Dodecahedral or Dyad Axes, because a revolution 

 around each of them brings the cube twice similar faces twice 

 into the same position in one revolution. Lastly, call the 

 third set of axes the Octahedral or Triad Axes, because three 

 times in each revolution about one of them the similar faces 



