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Ellcry W. Davis 



plainer than words. 1 The conic (it happens to be an ellipse) 

 through (o, i), (i, o), (o — i), ( — i, o), and tangent at these 

 points to lines to (oo, o) and (o, oo ) has the equation, in the 

 chosen coordinates, x 2 -f- y 2 = I ; is, indeed, the projection of a 



co.Y 



X-I-j' 



Fig. 33. 



circle, while O, the pole of 3 with respect to the conic, is the 

 projection of that circle's center. Let us call the conic C 2 . Var- 

 ious red vectors belonging to the I- and /-rays through O are in- 

 dicated. They are projections of elements of the circular rays 

 through the center of the circle. The two blue curves, portions 

 of which are drawn, are supplementary curves of the conic, 

 and projections of supplementary curves of the circle. The 

 perimeter of the conic, defined as fy/dx 2 -f- dy 2 , is 2tt. The arc 



1 For greater detail see Young, Theory of Point Sets, Chapter 2. 



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