How Well Did Ptolemy Understand Refraction?

Ptolemy does not discuss the simple lens in his Optics. He does have a good understanding of refraction and used a quadrant (Bk V, 7-18) to measure the angles of incidence and refraction at the air/water and air/glass surface. And although he discusses the refraction of simple curved surfaces he does not mention the focusing power of lenses or the creation of images by them. In his Optics, Bk V, 23-30 Ptolemy treats the refraction of the atmosphere as taking place at the boundary between the atmosphere and the aether beyond it. He notes that stars closer to the horizon are refracted more and that the apparent altitude is higher than if no refraction occurred. He also discusses the increase in the apparent size of an object when viewed from a denser medium.

Knowledge of the anatomy of the eye and the action of the crystalline lens does not appear until slightly later in Galen's time. Ptolemy's discussion of visual perception is found in Optics, Bk II. Among the topics he mentions are the eye's optical axis, its cone of perception, stereoscopic vision, the focus of attention and the perception of distance. While Ptolemy talks about object and image there is no discussion of how the eye itself works.