406 Proceedings of the Royal Physical Society. 



A^ towards b, and from the corresponding points opposite, 

 describe arcs intersecting those already drawn to represent 

 the angle a. The points so determined are the poles of the 

 crystallographic axis h (010). 



We have, finally, to project the crystallographic axes a 

 and h on to the primitive. To do so draw lines from these 

 poles to c (not to c), and the points where these lines 

 intersect the primitive are the positions, respectively, of the 

 a and h from which the measurements given in all the text- 

 books are taken, a, h, c being thus determined, the positions 

 of the remainder of the leading forms may be laid down, if 

 desired, by means of the spring dividers, from any table of 

 angles, and the various zones connecting these points may 

 be drawn upon the ball in the manner previously described. 



With the poles and zones in a concrete form before one, 

 the projection of a stereogram, or of -a gnomonogram, may be 

 rendered much simpler than could be at all possible without 

 this aid. 



Construction of Maps of Anorthic Crystals. — In proceeding 

 to draw the maps, it is a good plan to lay down the 

 working lines for the projection upon two or more separate 

 sheets of paper, and afterwards to transfer the points 

 and lines so determined to another sheet, which is to be the 

 fair copy of the complete map. It is well to have plenty 

 of margin around each of the working maps, and to choose 

 as large a radius as convenient. Any dimensions between 

 2J inches and 5 inches radius may be selected. The primi- 

 tives must, of course, be all of exactly the same size. 



1. The projection of c (001). — Draw two diameters at right 

 angles to each other, to serve as axes of reference, mark the 

 extremities of the front-and-back one xx^, those of the right- 

 and-left one YY^, and mark the centre z : the sphere in this 

 case is supposed to be viewed from above, or in the direction 

 of oz, the axis of the prism zone, or vertical axis of reference. 

 It is customary to draw the a axis in Anorthic crystal maps 

 parallel with the right and left sides of the paper. Therefore 

 the points where xx\ regarded as of indefinite length, cut 

 the primitive, will serve for the projection of the poles 



