The most important work ever produced in the field of physiological optics, this classic is a model of scientific method and logical procedure, and it remains unmatched in its thorough and accessible approach. This is the second in a three-volume republication of the definitive English translation of Handbuch der Physiologischen Optik, originally published by The Optical Society of America in 1924 and containing everything that was known about physiological optics up until that time. The substratum consists of the data that Helmholtz furnished in the two nineteenth-century German editions that appeared during his lifetime. These volumes also contain extensive supplementary matter that Nagel, Gullstrand, and Kries incorporated in the third German edition of 1911, as well as significant new material prepared for the 1924 English translation by C. Ladd-Franklin, Gullstrand, and Kries, with copious annotations by James P. C. Southall that brought the work up to date with current research. The first volume in this series explores the dioptrics of the eye; Volume II examines the sensations of vision, including stimulation by light; simple and compound colors; intensity and duration of sensation of light; and variations of sensitivity and contrast. Appendixes cover later findings on adaptation, twilight vision, and the duplicity theory; normal and anomalous color systems and theories of vision; and the nature of color sensations. The succeeding volume considers perceptions of vision.