327] ON THE STEREOGRAPHIC PROJECTION OF THE SPHERICAL CONE. 109 
may be considered as the stereographic projections of three great circles of the sphere. 
In fact suppose, as above, that the projection is made on the plane of a great circle, 
and calling this the principal circle, the projection of any other great circle meets the 
principal circle at the extremities of a diameter of the principal circle. It follows 
that the theorem will be true, if, given any three circles each two of which meet, a 
circle can be drawn meeting the given circles, each of them at the extremities of a 
diameter of the circle so to be drawn. It is easy to see that the required circle 
has for its centre the radical centre (point of intersection of the radical axes) of the 
given circles, and that the radius is the ‘Inner Potency’ of the point in question in 
regard to each of the three given circles. In particular the three circles having for 
centres the vertices of an equilateral triangle, and the side for radius, may be con 
sidered as the stereographic projections of three great circles of a sphere. This is a 
very ready mode of delineation of a spherical figure depending on three great circles 
of the sphere. 
2, Stone Buildings, W.G., March 21, 1863.