Francisco de Melo's theory of vision.

As a complement to his version of Euclid's Optics, the Portuguese mathematician Francisco the Melo (1490-1536) wrote a short treatise on the principles of vision that also included a summary description of the components of the eye. Combining arguments of geometric and anatomical nature, that text helps us to understand some of the conflicting ideas of the Renaissance period, before Kepler, but it also invites us to reread the contributions of a number of authors from the Middle Ages, with a particular focus on Alhacen and Witelo. Although Melo's theory of vision cannot be classified as exactly groundbreaking, it presents many interesting and singular peculiarities. In particular the way in which Melo justifies the existence of rays emitted by the eye while incorporating, at the same time, some elements of the intromission theories, and the clear steps he gives to single out the perpendicular ray in the mechanism behind a faithful and distinct vision.