In its development of instruments, methods, and standards for measurements made by the human eye, the National Bureau of Standards has obtained information on the response of that physiological mechanism to stimulation by radiant energy. Sometimes this information is specifically sought, but more often it is incidental to another purpose. This article shows how the need for this information has arisen, recalls some of the most important studies of physiological optics carried out at the National Bureau of Standards, indicates the applications made of their results, and explains briefly the purposes of current research. It is convenient to discuss physiological optics under the headings: evaluation of light-dark patterns, light measurement, color measurement, color differences, and color perception.