Dentur puncta quinq; A, B, C, D, P. Ab eorum aliquo A ad alia duo quævis B, C, quæ poli nominentur, age rectas AB, AC hisq; parallelas TPS, PRQ per punctum quartum P. Deinde a polis duobus B, C age per punctum quintum D infinitas duas BDT, CRD, novissime ductis TPS, PRQ (priorem priori & posteriorem posteriori) occurrentes in T & R. Deniq; de rectis PT, PR, acta recta tr ipsi TR parallela, abscinde Page 80 quasvis Pt, Pr ipsis PT, PR proportionales, & si per earum terminos t, r & polos B, C actæ Bt, Cr concurrant in d, locabitur punctum illud d in Trajectoria quæsita. Nam punctum illud d (per Lem. XX) versatur in Conica Sectione per puncta quatuor A, B, P, C transeunte; & lineis Rr, Tt evanescentibus, coit punctum d cum puncto D. Transit ergo sectio Conica per puncta quinq; A, B, C, D, P. Q. E. D.