S. Z. Penfield — Stereographic Projection. 



employed either in semicircular or rectangular form as 

 nary protractor for laying off and measuring plane ang 



an ordi- 



les. 



Scale No. 3, accompanying the graduated circle, figure 3, 

 repeats the graduation of the base'line of the protractor, and 

 the graduation is further continued for arcs of more than 90° 



