in the Study of Crystallography 327 



equatorial circle representing face No. o of the crystal. 

 Mark on it the nodes of the meridian. The one correspond- 

 ing to o on the equator of the metrosphere is node No. o, 

 from which azimuths are measured. The diameter connecting 

 nodes No. o and No. (o, i') represents edge No. (o, i) which 

 face No. i makes with No. o. From the specification of 

 the crystal the angle of inclination of these two faces is a 

 right angle. Therefore in its original position the meridian 

 of the metrosphere coincides with great circle No. i. Let it 

 be drawn. The third independent face, No. 2, which we 

 find on the crystal is inclined to each of the other two at a 

 right angle. With the metrosphere in its original position, 

 clamp the movable quadrant at 90 of azimuth, the quadrant 

 coincides with circle No. 2. It is generally more convenient 

 to bring the meridian to coincide with the great circles to be 

 drawn, because then the complete semicircle is drawn at 

 once, and both nodes are marked at the same time. For this 

 purpose the metrosphere is rotated from its original position 

 round the axis of the equator, which remains coincident with 

 circle No. o, until division 90 of azimuth coincides with 

 node (o, i'). The meridian now corresponds with the position 

 of circle No. 2. Let it be drawn. Circle No. 2 obviously 

 cuts No. i at right angles, which can be at once verified by 

 measurement of the arcs between node (1,2) and nodes (o, 2) 

 and (o, 2'). If we attempt to place any of the three remaining 

 faces, we find that its place is already occupied by one of 

 the circles, o, i, or 2, to which it is parallel. If we now wish 

 to specify the crystal in terms of its central representation on 

 the sphere, we have, for the inclination of the faces o to i, 

 90 ; o to 2, 90 ; and i to 2, 90 ; and, for the position 

 and direction of the edges : edge No. (o, I ) azimuth o, 

 altitude o ; edge (o, 2), (90, o) ; and edge (i, 2) azimuth 

 indeterminate, alt. 90. The plane angle made on any face 

 by any other two is equal to the arc on the corresponding 

 circle contained between the nodes made by it on meeting the 

 two other corresponding circles. In the present case they are 

 found by measurement, as they are seen by inspection to be 

 right angles. 



