414 Proceedings of the Royal Physical Society, 



first projection, and draw lines from these both ways through 

 c. Beyond this the procedure is the same as that already 

 described. 



The Linear {or Quenstedfs) Projection of Anorthic Crystals. 

 — A method of projection much in use amongst crystallo- 

 graphers of the Continental school is that with which the 

 name of Quenstedt is associated. All things considered, it 

 is not so generally useful as the stereogram ; but it is un- 

 doubtedly a valuable adjunct to that projection, and is useful 

 in many other ways. Moreover, it is easily constructed. A 

 short description of the principles upon which it is con- 

 structed may advantageously be given here. The plane of 

 the map is parallel to some one face of the crystal to be 

 represented — usually to one of the pinacoids, and most com- 

 monly is parallel to the prism zone. The different faces are 

 represented, not by their poles, but by their respective traces, 

 which are supposed to be of indefinite length, and they are 

 all conceived to pass through the axis perpendicular to the 

 plane of projection at the parametral length proper to the 

 species. As directions, and not magnitudes, are to be repre- 

 sented, every face is supposed to be shifted parallel to itself 

 until it cuts the parametral point referred to. In accordance' 

 with this principle, the face a (100) is shifted parallel to 

 itself so as to pass through the point in question. So, too, 

 is h (010), and m (110), and all the faces of the prism zone. 

 Their traces are prolonged indefinitely, as it is the directions 

 of the faces that are delineated. The parametral length 

 of h is drawn upon the line which represents the direction of 

 a ; and, in like manner, the parametral length of a is drawn 

 upon the line whose direction is that of b. Next, supposing 

 that the plane of the projection is that of the prism zone, the 

 orthodomes and clinodomes (or macrodomes and brachydomes) 

 have to be drawn in. Taking the unit forms first : — These 

 cut the a axis, or the b axis, as the case may be, and also the 

 c axis, each at unit length, their traces will be at unit dis- 

 tances from the centre of the projection — the macrodomes at 

 the unit length of a and parallel to b, and the brachydomes 

 at the unit length of b and parallel to a. 



Next we have to consider the pyramids. The unit 



