44 



Trans. Acad. Sci. of St. Louis. 



When a = 30°, c = B, and the focus is on the circumfer- 

 ence of the circle. 



(This ellipse is the common isometric projection of the 

 circle. ) 



Fig. 2. 



To determine common points of ellipse and circle we have 



x'^ -^ f = i?2 



x^ ?/^ 



1 + sin a 1 — sin a 

 whence 

 and 



= i?2 



referred to OX, OY, 



X = :h B COS 6 



y = zk R sin 0. 

 These two equations give us the points -4, B, p, q, (Fig. 2), 



