Sentence:  In this case, to which Sphero-Cartesians, and all other sphero-quadrics, whose <WSD>dirigents</WSD> are small circles, are analogous in Spherics, the dirigent conic "F" must also have double contact with "S". 
Meaning of the word marked with <WSD> tags:  The line of motion of a describent line or surface that in so moving defines a plane or solid figure; a directrix.