A DRAWING-BOARD WITH REVOLVING DISK FOR 

 STEREOGRAPHIC PROJECTION 



ALBERT JOHANNSEN 



The University of Chicago 



Professor Wulfing recently showed the writer a wall chart for 

 stereographic projection which he has since described. 1 It consists 

 of a ground glass plate back of which is pivoted a 70 cm. Wulff net 

 which is made of pasteboard and projects beyond the glass cover 

 so that it may be turned to any desired position. The advantages 

 of using the Wulfing chart were so apparent that the writer has 

 constructed a drawing-board, for the individual use of students, on 

 a somewhat similar plan but combining with the Wulff net a half- 

 net with the north pole at the center. The construction is simple 

 and the board inexpensive. 



In making stereographic projections by ordinary methods, one 

 must either work out his own dimensions, use a Penfield protractor, 

 or a net like that of Fedorow or of Wulff. Transparent nets are an 

 improvement over Penfield's method, although one must always 

 carefully center the net for each measurement. With the drawing- 

 board here described, the net is revolved instead of the paper and 

 no centering is necessary. 



The board was constructed from an ordinary drawing-board, 

 332X43I cm. in size. It was placed on a lathe and a recess, 22 cm. 

 in diameter (D-J in the illustration), was turned out halfway 

 through the board which was 2 cm. thick. A further slight cut 

 (H) was made to reduce friction when the disk is rotated, leaving 

 only the bearing shown at F-F. A 2 cm. hole was turned entirely 

 through the board (P-G). To keep the dial disk (B-C) perfectly 

 flat, it was made from a piece of three-ply, built-up pyrography 

 board, such as is sold in all art stores. This disk, also, was accu- 



1 E. A. Wulfing, "Wandtafeln fur stereographische Projektion," Centralbl. f. Min., 

 Geol., u. Pal. (1911), 273-75. 



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