Solution of Aistronomical Problei 



67 



I 



which h cuts the sphere. Great circles are projected into 

 circle? cuttino- s at the ends of a diameter, and small circles 

 are also projected into circles; while angles are projected into 

 equal anole?. The stereooraphic projections required are 

 very much more ra])idly and easily sketched if they are drawn 

 on tracing-cloth (Herr G.Wulflf ^' recommends " Pauspapier ^'), 

 and use is made of the " stereographic net/^ This may be 

 described as the stereographic projection of lines of latitude 

 and longitude on a plane through a line of longitude. Wulff ^ 

 has given an accurate diagram in which every other degree 

 is shown and a full description of this net, so that it is un- 

 necessary to give a careful drawing here or to enter upon the 

 details of its use. For purposes of illustration, however, a 

 rough diagram is added in which every tenth degree is shown 

 (fig. 1). The diameter of the net is equal to the diameter of 



the circle s of the projection and the centre of the projection 

 (drawn on the tracing-cloth so that the net can be seen 

 through it) is always placed over the centre of the net. 

 Mr. S. L. Penfieldf describes various kinds of stereographic 

 nets and similar contrivances for drawing stereographic pro- 

 jections, but the stereographic net described by AVulff (and 



* Zeitschr.f, Kryst u. Mm.xxx\\. 1, p. 14(1902). 

 t American Journal of Science, xi. 1901. 

 F2 



