Claudius Ptolemy and Giambattista Della Porta: Two Contrasting Conceptions of Optics

We address the phenomenon of reflection in concave spherical mirror in two contrasting approaches to optics. In his Optics (ca.165) Ptolemy applied the cathetus principle as a regulative means for explaining qualitatively effects related to visual perception in concave spherical mirror. By contrast, Della Porta’s study of reflection in concave spherical mirror in Bk. 17, Ch. 4 of his Magia naturalis (1589) and De refractione (1593), was based on the assumption that there is a reciprocal relation between reflection in concave spherical mirror and refraction in glass sphere. We juxtapose these two studies and draw several philosophical lessons from the comparison between these two practices with a view to throwing into relief the fundamental differences in their respective conceptions of optics.