Begins with a brief account of Giambattista Della Portäs life, a review of the genesis of De Refractione, and an examination of the textual sources on which Della Porta drew in composing the book and its analytic narrative. Explains the why and how of this edition and translation of the original Latin text. Examines Della Portäs physical account of refraction, his overall account of vision, his account of visual illusions and their environmental or pathological causes, and his analysis of the rainbow and some other meteorological phenomena. Addresses the historical significance of Della Portäs account of light and sight. The facing Latin-English edition that follows contains biographical sketches of authors cited by Della Porta. Illus.