Simpler Methods in Crystallography. 351 



to draw a line at right angles to the oC axis in vi. from the c 

 on the oz axis, and from this foreshortened C draw a line to 

 the point where OA cuts the primitive in vii. A perpen- 

 dicular to this line drawn outward from o is to be signed E, 

 and e where it cuts the primitive. Then from the opposite 

 point where oa cuts the primitive, which is the point of sight 

 in this case, a line to e will project the required position of e 

 (Oil) upon 0(7. Draw parallels at any convenient distance 

 outside of the primitive to 0(7, OA, and measure off proportional 

 lengths upon these, as in the former cases. From these points 

 so determined draw lines radial to o to cut the primitive, 

 and from these, in their turn, draw lines radial to the point 

 of sight, and where they cut oC is the position required. 



62. In each of these cases a study of Figure 3 will further 

 elucidate the method. It appears tedious and complicated 

 when expressed in words and in a non-mathematical form ; but 

 in actual practice no real difficulty ought to be met with. 



63. The remainder of the construction has been already 

 described, and consists of little more than drawing two arcs 

 of great circles each through three given points,. which by 

 their intersection give the positions of the poles required. 



64. It is sometimes convenient to employ a map of a 

 monosymmetric crystal drawn on the c projection — on the 

 whole I prefer this. The usual objection urged against it is 

 that it is more difficult to construct than the h projection, 

 and that it does not show the monoclinic character quite so 

 clearly. These objections seem to me to possess but little 

 weight. One way to do is to make use of loth ; and in the 

 maps I have drawn for " The Mineralogy of Scotland," of 

 which an example is appended to this paper, the advantage 

 of this plan will, I think, be sufficiently manifest. 



65. We may conveniently choose Hornblende as the species 

 to be mapped on the c projection. For this purpose draw 

 a circle of convenient radius as before, and draw in diameters 

 at right angles to each other, marking their extremities 

 (which may be of any length outside the primitive), x in 

 the case of the front and back axis, and Y in the right 

 and left, and the centre o. From o as a centre, with a 



