410 Proceedings of the Boyal Physical Society. 



tions through which arcs of circles are drawn from bb^ to 

 determine the position of the unit macrodomes (101), (101). 

 To lay down the other macrodomes, a similar plan is adopted 

 to that already described. Lines outside the primitive are 

 drawn respectively parallel to aa^ and the inclined crystallo- 

 graphic axis c (001), and proportional parts are measured off 

 as already described and illustrated. Lines are drawn 

 thence radially to the primitive, are projected to the front- 

 and-back axis of reference ox, ox^ from Y, and arcs of circles 

 through bh'^, and these points define the position of the re- 

 quired positive and negative macrodomes on ac. These in 

 Ehodonite are /x (401), and r (201), 4> (401). 



4. Projection of the Poles of the Brachydomes. — It happens 

 that in Ehodonite only one brachydome has been recorded, y 

 (041) ; but as this species is in many other respects a repre- 

 sentative one for the purpose, the mode of construction may 

 as well be fully described. Much of the foregoing descrip- 

 tion will apply equally to this. The inclination a of 6 to the 

 vertical axis of reference is 103° 18' T, or 13° 18' 1" more 

 than a right angle. To draw the first steps in the projection, 

 we have to assume that we are viewing the sphere of projec- 

 tion from the front in the direction of aa)-. The crystallo- 

 graphic (or inclined) axes c and b are laid down at right 

 angles to each other as before, and inclined to the axes of 

 reference as if rotated through 13° 18' 7" ('231 of the unit 

 measured as a chord of the primitive). Then, on the vertical 

 axis of reference oz, '62127 is measured off on either side 

 from for cc^, and lines are drawn from these points to the 

 inclined crystallographic axis hb'^ at its full length, and these 

 are prolonged to cut the primitive. Perpendiculars through 

 these intersections are drawn through the origin o, and pro- 

 longed to the length of the radius beyond the primitive. 

 The points so determined are projected on to the rectangular 

 axes of reference OY, OY^, and arcs of great circles through 

 these from aa^ give the position of the poles of the unit 

 brachydomes by their intersections with bc¥. For the 

 other brachydomes, lines are drawn outside the primitive, 

 parallel respectively to lines joining bb^ and the inclination 

 of the oc crystallographic axis as determined by the angle a. 



