426 Proceedings of the Royal Physical Society. 



right horizontal axis of reference o5 = OBXsin 7. From h 

 draw a line parallel to OA, and from a draw another parallel 

 to OB, and through the point of intersection of these lines, 

 which call D, draw a line dod^ to the left. On OD^ mark off 

 a distance equal to OD, and draw a line through that point 

 parallel to oc. 



We have now obtained the skew denoted by the angle 

 7, but the axial cross so far is only duoclinic, whereas it 

 has to be triclinic. We have therefore now to draw the 

 clinographic projection of the axis last described as further 

 modified by the angle «, which is that which it forms with 

 the vertical axis of reference. ITow, therefore, lay off on oc, 

 oc^ = oc cos a, and from the position of c^ thus found draw a 

 line parallel to dod^, and then a line drawn through the 

 point of intersection and gives the desired inclination and 

 unit length of the oh axis, which is taken as unity, and has 

 not therefore to be proportioned to be adapted to the para- 

 metral length proper to the species under treatment. 



Given these axial crosses, the next step is to adapt the 

 principle of the Linear Projection to them. The reader has 

 therefore to conceive that the Qiienstedt's Projection, already 

 described on p. 414, instead of being drawn on a plan, is 

 projected so as to pass through the centre of the axial cross, 

 and to be skewed to the left and tilted forward at the same 

 angles as those of the horizontal arm of the axial cross. Upon 

 this, which forms the middle plane of the construction, the 

 elementary forms of the crystal are to be carefully drawn for 

 use as standards. The six pinacoidal faces, and the unit 

 pyramid, as well as other forms of common occurrence, are 

 drawn in. From the principle of construction of this pro- 

 jection described already, it will be not at all difficult to 

 delineate the true direction of the intersection of any two 

 planes in any position of the clinographic drawing. It will 

 probably give a beginner a certain amount of trouble to work 

 the method out for himself, but it is certainly worth while 

 to do so. 



In order to illustrate the methods of work followed in the 

 next stage, a description of the steps in the construction of 



