Simpler Methods in Crystallography. 411 



Proportional parts measured along these lines enable one to 

 determine the positions of the poles required. 



5. The Projection of other Forms. — No real difficulty need 

 be experienced in projecting the poles of any brachypyramids 

 or macropyramids if these instructions are carefully followed 

 with reference to the figures already given in the former 

 paper, and with the solid construction before one. Arcs of 

 circles are drawn from the primitive to pass through c, and 

 are intersected by other arcs passing through hh. Taking z 

 (16.2.3) in Rhodonite for an additional example to those 

 already given, an arc from (810) on the primiti^^e is drawn 

 through c and is intersected by another arc passing through 

 the pole of the macrodome (16.0.3). a (4.1.12) again is got 

 by the intersection of an arc from (410) through c with 

 another arc through the macrodome (103). 



These details may seem very complicated, and not at all 

 in accordance with the instructions given in the text-books ; 

 but they are, in practice, much more simple than they appear 

 in description. One is occasionally inclined to suspect that 

 some of the writers of these text-books have never attempted 

 the construction of these maps themselves ; otherwise, if they 

 really wished students to know how to go to work, they 

 would have given clearer instructions for the purpose than 

 they have done. 



• 



Anorthic Gnomonograms. — Gnomonograms as applied to the 

 illustration of crystals belonging to other systems have been 

 referred to in the First Part of this paper. It only remains 

 to add now that the same system of projection is equally 

 applicable to Anorthic Crystals. The best pole for the 

 tangent plane to touch the sphere of projection is usually 

 that of c (001). As an expeditious and simple method of 

 determining the zonal relationships of the various poles this 

 method has no equal, and I now invariably make use of it for 

 that purpose as an invaluable adjunct to the stereograms. 

 Where the poles are situated in positions too widely separated 

 to be shown on a map of any convenient dimensions, two (or 

 even more) maps may be advantageously used, each projected 

 on a tangent plane suitable for the purpose in view. The 



