404 Proceedings of the Royal Physical Society. 



metasilicate. The elements of the crystal as accepted by- 

 Dana (Sixth edition, " Syst. Min.," p. 378) are as follows:— 

 a '.h : c= 1-07285 : 1 : '62127; ct = 103° 18' 1"; y3=108° 

 44' 8"; 7 = 81° 39' 16". 



The Construction of Solid Projections of Anorthic Crystals. — 

 In commencing this work, which is fraught with far greater 

 difficulties than the descriptions hitherto published would 

 lead one to suppose, it is as well first to attack the problems 

 by concrete methods. For this purpose, therefore, take a 

 ball of soft wood : a convenient size is 5 inches in diameter, 

 which is the size I have used for the purpose in the Edin- 

 burgh Museum. A good pair of spring dividers is likely 

 to be of much use, and one of the points of this instrument 

 should be bent inwards about 25 degrees, and both of them 

 kept as fine as possible. The other requisites are a table of 

 chords, and a good diagonal scale to 2J inches as a unit. 

 The preliminary step consists in marking three great circles 

 on the ball exactly at right angles to each other. For this 

 purpose select any point on the ball, and from this as a 

 centre, with the radius 1*414 taken from the diagonal scale, 

 describe on the ball a diametral line. From any four points 

 in this, about at right angles to each other, describe arcs at 

 the opposite pole, so as to mark the centre of the sphere 

 opposite the one first selected. Then, from this, check the 

 first diametral line, so as to eliminate any possible errors. 

 Next, with the same radius, draw another great circle 

 passing through the poles already determined, and from the 

 points where this circle intersects the first draw the third 

 great circle, afterwards finding its second pole and checking 

 again, as before. Mark one pole c, and the one opposite 

 it c^. Also mark the other poles in like manner A, A^, and 

 B, B^ respectively. The circle aba^b^ is to be the primitive, 

 and cc^ are the poles of the axes of reference. We may 

 conveniently refer to all three circles as the great circles 

 OF reference, to distinguish them from the crystallographic 

 zones. 



These poles of reference need not all, necessarily, coincide 

 with the poles of any crystallographic faces in the Anorthic 

 System, though they may do so in a few cases. 



