Simpler Methods in Crystallography. 



331 



as being as close as an ordinary geometrical draughtsman 

 can practically work ; and assuming that quadrant iv. in the 

 circle of projection represents the sphere as seen from its 

 side at h, we measure off on ox a distance '845, and on 

 oz we set off 1-313. With a fine pointed pencil and a 

 straight-edge these two points are joined, and the line pro- 

 longed both ways across the circumference of the circle. 

 Then from the points where this line cuts the circle, with 

 any convenient radius, two intersecting arcs are described, 

 and a line is then drawn from the point of intersection to o. 

 Next, at right angles to this line, draw both ways a line 

 touching tlie circle of projection and cutting oz, ox. Sign 

 these points by any arbitrary symbol, say the one on ox with 

 a capital A, and the one on oz with a capital c. 



24. If the ball with -the sheet of glass tilted against it be 

 referred to, it will be seen that c corresponds to the point on 

 the sheet of glass where the vertical axis touches the glass, 

 and that A represents the nearest point where the foot of the 

 sheet of glass approaches the ball along that line. Eight 

 and left of the point on the glass corresponding to A, it will 

 be seen that the glass is farther away still. We now require, 

 therefore, to determine the position of these points more 

 exactly. With this end in view, set off a quadrant of a 



