1.2 - Practical Optical Knowledge of Mirrors and Lenses in the 16th Century
In the 16 th century, texts on the perspectivist tradition of optics were widely diffused in printed form. In Italy they were the subject of original studies by mathematicians and "natural magicians".
The Venetian Ettore Ausonio (c. 1520 - c. 1570) made the first attempt to integrate the problems of the position of the burning point and the formation of images in a concave mirror. Ausonio determined that the burning point in this kind of mirror coincides with the point at which the orientation of the image visible in the mirror changes. The Neapolitan Giovanni Battista della Porta (c. 1535 - 1615) extended the concept of concave mirrors to convex lenses. The burning point of a convex lens coincides with the point of inversion of an image, as well as with the point at which the eye perceives the image at maximum enlargement.
The concept of the "inversion point" identified by Ausonio and Della Porta went beyond the scope of the perspectivist tradition, through the assimilation of concepts pertaining to practical optics. This corpus of concepts, which played a fundamental role in transforming optical theory, should however be held distinct from the material knowledge that was indispensable for constructing mirrors and lenses for eyeglasses.
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