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1. The spectral sensitivity of red-green dichromats was determined using heterochromatic flicker photometric matches (25-30 c/s) on the fovea. These matches are upset after a bright bleach and consequently the spectral sensitivity is altered.

2. Preliminary experiments indicate that under the conditions in which these experiments were performed, the blue cone mechanism of deuteranopes and protanopes cannot follow 20 c/s flicker. If dichromats lack one of the normal pigments then the upset of these matches monitors the change in spectral sensitivity of a single mechanism.

3. After a bleach which removes all the cone pigments, the spectral sensitivity recovers with the time course of pigment kinetics as measured by densitometry.

4. An intense background also changes the relative spectral sensitivity of the dichromats. On real equilibrium backgrounds, the changes in spectral sensitivity follow those predicted by the pigment changes measured by densitometry. The predicted changes are obtained by modifying the Rushton equilibrium equation to take into account the density of pigment.

5. The relationship of these changes to the luminance of the background is independent of the colour of the background light.

6. In contradistinction the effect is dependent on the colour of the lights which were flickered. These experiments indicate that a narrowing of the spectral sensitivity curves takes place on both sides of the dichromats' _max.

7. The change in relative spectral sensitivity as a function of background intensity was also determined by increment threshold measurements. These changes can be expressed in terms of deviations from Weber's law (I/I = const.) if I and I represent the number of chromophores destroyed by the test and background.

8. The relative spectral sensitivity of the dichromat was changed by decentering the point of pupil entry. This upset was abolished by bleaching. The size of the upset was correlated with the magnitude of the S—C I effect.

9. Given the hypothesis of pigment density (self-screening), the results of expts. (3)-(8) are consistent and allow the calculation of a maximum optical density for those pigments which underlie the dichromats' long-wave mechanism. For the deuteranope a D_max of 0·5-0·6 is calculated and for the protanope a D_max of 0·4-0·5 is obtained.