The present system of colorimetry, dating from 1930, was early found (by Judd, Wright, and Stiles — three of its formulators — for example) to have serious deficiencies, despite its enormous usefulness. Astonishingly little incisive examination of these deficiencies, their characteristics, and possible cures has been published. We use direct spectroradiometry of the stimuli from the traditional bipartite field of a visual colorimeter, several primary-sets, and visual matching of narrow-band, multiple-band and broadband lights, to elucidate'the differences between normal human vision and the mathe-matical construct called the Standard Observer. Results: 1. Large errors in computed chromaticity, with pairs of highly-metameric lights pronounced to match exactly by a normal human observer, when there is strong content in one or more of the following spectral regions: violet, blue-green near 500nm, yellow near 570nm, or in the deep-red. 2. Agreement is sometimes poor between visual definition of complementary lights, and definition by means of the color-diagram. 3. Color diagram chromaticity coordinates are defined: x=X/(X+Y+Z). The denominator is not representative of "total perceived brightness" and so is not a legitimate element in the construction of a proper color diagram. In addition, the traditional method of construction simply fails, with some sets of real primaries. 4. Perceived-brightness-per-watt of mixtures can be doubled, at constant color, by choice of components. 5. Spectral content in the neighborhood of 500 nm sharply reduces the perceived brightness of white-light mixtures. 6. Although stringent tests of Grassmann's "Additivity Law" show it to hold visually, still "transformation of primaries" and "normalization of color-matching functions” appear never to have been legitimate.
7. Spectral regions near 450 nm, 530 nm and 610 nm appear to excite maximum visual response per watt.