some Carbonates of the Calcite Group. 



323 



distance from the crystal to the plate, CO, 

 into tan 20. BC, the distance of the projec- 

 tion of the plane producing spot A from the 

 center C, is equal to the distance CO, the 

 radius of the sphere of projection, multi- 

 plied by cot 0. Then by the use of a straight 

 edge and a table or special scale connecting 

 tan 20 and cot 0, it is possible to record the 

 distance of a spot from the center and to 

 locate its position on the projection with a 

 single setting. 



The pattern may be reproduced in the cen- 

 ter of the paper in a number of ways. 

 Either of the following two has been found 

 convenient. The photograph may be made 

 on X-ray film instead of plate. This film 

 is mounted on the paper and the positions 

 located on the projection by directly prick- 

 ing through the film with a sharp pin. This 

 is the quicker method but it destroys the 

 film. It is much better to make a contact 

 print from the photograph and to prick 

 through this print. 



For drawing the projection from this re- 

 production a ruler carrying a special scale 

 is most convenient. It is desirable that this 

 ruler be pivoted on a pin through the draw- 

 ing table, so as to be moved on the paper 

 about an axis through the center of the pro- 

 jection. Such a ruler is shown in fig. 5. 

 Of course, a different scale will be required 

 for each distance of crystal to plate, but 

 since distances of 3, 4, and 5 cm., usually the 

 last two only, are all that are required in 

 ordinary practice, three or even two rulers 

 are sufficient. These rulers can be prepared 

 very simply from the tables I, II, and III. 

 It is best to use a sphere of projection of 

 the same radius at all times so that all pro- 

 jections may be comparable one with an- 

 other. One having a radius of 5 cm. is commonly used. 

 With this radius most of the projected spots that occur 

 will lie beyond the reproduction of the photograph in the 

 center, but still the projection is not too large to be con- 



) 



* §3 



rt .§ 

 «h hi 



9 ►» 



03 p) 



r- 5. rH 

 O T3 



PI W 



p-s 



So* 



• S ^ 



P-i 03 

 03 -— ' 



id ft 



£.§ 



ft 



