412 Proceedings of the Royal Physical Society. 



method of construction is soon mastered ; and, as all the 

 zones are shown by straight lines, many of the difficulties 

 attendant upon the construction of stereograms are entirely 

 avoided. It seems to me so much more convenient and 

 practically useful to have as many zones as possible 

 delineated so as to be easily read by inspection than it is 

 to have to calculate them out — simple though the method of 

 calculation referred to undoubtedly is. It may not be out of 

 place here to remark that the methods in question are mostly, 

 with great precision, set forth in the introductory chapter by 

 Miller in Brooke and Miller's "Mineralogy" — one of the 

 most valuable works on the subject in the English language, 

 even yet. 



As an example of a gnomonogram of an Anorthic crystal, 

 the best that can be selected is either Chalcanthite (755), the 

 hydrous cupric sulphate, or else Axinite (410), the aluminium 

 and calcium boro-silicate. Each of these represents a type 

 of anorthism widely different from the other. 



Chalcanthite is best projected on a tangent plane touching 

 the sphere of projection at the pole of (101). For a gnomo- 

 nogram to a sphere of 2J inches' radius a sheet of paper 

 about 3 feet wide by 18 inches across is required. The exact 

 details of the earlier steps in the projection can easily be 

 learned from a study of the description already given in the 

 former Part in connection with this method of projection as 

 applied to other systems. The circle representing the sphere 

 in the first projection, and the various construction lines 

 required in connection with ic, may be drawn in the top 

 left-hand corner of the paper. When this has been done, 

 mark the position of (101) by a cross about two-thirds 

 towards the front of the middle of the paper. Eule a line 

 through this to the opposite edges of the paper for the zone 

 of the macrodomes, a-c ; and another at right angles to it 

 extending to either side. Then take off the distances of a 

 and c from (101) from the first projection, and sign them in 

 the usual way. Draw a line through a at right angles to ac, 

 and upon this mark off the positions of (110) and (110) 

 taken from the first projection, and sign these respectively 

 M\ m}. Then at c set off a line towards the left, forming the 



