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1. Retinal densitometry has been used to determine the optical density of erythrolabe in foveal cones.

2. A self-screening method was used, as in Rushton's (1963b) measurements on chlorolabe.

3. Psychophysical experiments have demonstrated that self-screening in cones depends on the direction of the light in the same manner as the Stiles—Crawford efficiency. Stiles—Crawford measurements were therefore made on each subject and a method was devised to allow for the corresponding reduction in self-screening in densitometry.

4. Complications arising from coloured bleaching products, the presence of a second visual pigment and the fluorescence of visual pigments are considered.

5. The mean value for optical density at 560 nm was found to be 0·40 ± 0·07 S.E. of mean for the five subjects (two deuteranopes, two deuteranomalous subjects and one normal subject).

6. This result is in good agreement with Rushton's (1963b) value of 0·35 for chlorolabe using retinal densitometry.

7. The densitometry results are also in reasonable accord with recent psychophysical and microspectrophotometry measurements. This provides evidence that densitometry measures all the cone pigment in an unbiased manner and not just a superficial sample.