Optical diagrams were part of the classical tradition of Euclid and Ptolemy's optics, but in the late Renaissance they gained a new epistemic significance as means to represent the functioning of recently developed glass mirrors and lenses. From the late 16th century onwards, these artifacts became available in rapidly increasing quantity and quality, whereby artisans, scholars and the general public were confronted with a very broad and confusing range of new optical experiences, such as images hanging in the air, enlarged or inverted.
The published and unpublished writings of the Neapolitan scholar Giambattista Della Porta (ca. 1535-1615) are the earliest extant evidence of prolonged and systematic attempts to formulate diagrammatic rules for refraction in glass spheres and lenses. We do not know how advanced Della Porta was in his own studies and how much he borrowed from artisanal traditions, but there is little doubt that he invested a lot of effort in searching for and communicating new ways of conceiving optical phenomena. Although the results of his efforts are often belittled in the literature, I will try to show, on the one hand, how difficult the task facing him was and, on the other hand, how much he contributed to shape the concepts of modern optics by transforming optical diagrams into paper tools capable of constructing new visual experiences.