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 2 + y 2 = B 2 



+ 



1 + sin a 1 — sin a 

 whence 

 and 



y 2 _ 7? 2 y referred to OX, OY, 



X = db B cos 6 

 y = ± B sin 0. 

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



