The contrast sensitivity of the colour mechanisms of the human eye.

1. To isolate the colour receptive systems of the eye, a sinusoidal grating of one colour was superimposed on a bright background of another colour and the threshold contrast for resolving the grating was determined.2. A procedure is described which allows one to estimate the extent by which the bright background reduces the contrast of the test grating of another colour. Using this procedure, the measured threshold contrasts of the test grating were replotted in terms of the effective contrast sensitivity for the composite target.3. The effectivity of a red background in reducing the contrast of a green grating was found to be determined not by its apparent brightness but rather by the extent to which it excited the green mechanism.4. The effective contrast sensitivities for a green grating on a green background, for a green grating on a red background, and for a red grating on a green background were found to be not significantly different.5. Visual acuity for a blue grating on a yellow background was reduced from normal by about a factor of six. This loss in visual acuity was due to both a reduction in contrast sensitivity and a reduction in resolution.