Reflection in a concave spherical mirror—two contrasting conceptions of optics: retrospective

We contrast two approaches to the optics of reflection in concave spherical mirrors. In the <i>Optics</i> (ca. 165), Ptolemy employed the cathetus principle as a regulative rule to account qualitatively for visual appearance in concave spherical mirrors. In his <i>Magia naturalis</i> (1589) and <i>De refractione</i> (1593), Della Porta's analyses rested on the assumption of a reciprocal relation between reflection in concave mirrors and refraction in glass spheres. These two historical cases highlight two fundamentally different conceptions of optics: Ptolemy explained optical phenomena based on vision, whereas Della Porta explained the same phenomena by appealing to the geometrical action of light. The shift from perception to the geometry of light rays marks an important development in the history of optics. The elimination of vision from the equation, as it were, ushered in modern optics.