  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.
