Visual contrast thresholds of the normal human observer were detd. for a wide range of field brightnesses. The observers were asked to detect the presence of a circular test patch which was projected onto the adapting field. Data were obtained for patches lighter and darker than the field, with two values of exposure time and various sizes of test patch. The contrast thresholds were detd. by the psychophysic method of constant stimuli involving 5 degrees of contrast for each threshold detn. The resulting data are summarized in tables and graphs based upon some 450,000 responses.