This book is a translation of the first volume of Y. LeGrand's three-volume treatise on physiological optics. It covers topics related to the image-forming structure of the eye and examines patients and fitting glasses, mixing the traditions established by Helmholtz and others. The book was translated from French to English by Professor Sami El Hage and published in 2013, making it the last of LeGrand's three volumes to be translated to English.