Nonadditivity in color matches with four instrumental stimuli

Nonadditivities in 10 degrees color matches have been studied with regard to a hypothesis about the effect of the Maxwell spot. Rod intrusion was eliminated by adding a fourth instrumental stimulus to the usual three stimuli of colorimetry and then determining the matches that were unaffected by attenuation with a rotating-sector disk. To avoid other instabilities from adaptation, the shortest wavelength used was 470 nm. The matches were made in a visual colorimeter, with interference filters as monochromators. The stimuli were measured directly with a calibrated photomultiplier, and further matches were predicted by assuming linearity. These matches were set up, and significant departures from Grassmann's laws were observed, which could be explained if the eye generally makes a compromise match over the whole field, regardless of any instructions to ignore the Maxwell spot. Matches made in a 2 degree field were more additive than those made in a larger field.