44 



Trans. Acad. Sci. of St. Louis. 



When a — 30°, c = R, 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 = R 2 



sr 



+ 



y 



1 + sin a \ — sin a 



_ V referred to OX, OY, 



whence 

 and 



x = ± R cos 6 



y = ± R sin 6. 



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



