2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/127675We give a new, elementary, purely analytical development of \textsc{Descartes}' theorem that a smooth connected surface is a perfect focusing lens if and only if it is a connected subset of the ovoid obtained by revolving a cartesian oval around its axis of symmetry.12 pages; proof of the exceptional cases to Drucker's theorem elaborated; one new importante reference is added; typos correctedGeneral MathematicsClassical Analysis and ODEs00A05text