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Foveal dark adaptation in 58 subjects and photopigment regeneration in 60 subjects from 10-78 years of age exhibit parallel slowing of recovery rate with increasing age, with significant correlation of the two functions among individuals. The data are suggestive of an initial slight decline in rate before age 50, followed by a greater decline occurring at different ages in different individuals. Longitudinal data for one subject from age 40-65 show an increase in pigment regeneration time constant consistent with this idea. Foveal sensitivity and photopigment density both decrease with increasing age and are significantly correlated among individuals, although sensitivity declines more with age than does photopigment density. In contrast to earlier proposals based upon the Rushton-Dowling equation, we found no universal constant of proportionality to relate log relative threshold to photopigment within our population.

It is ten years since it was demonstrated by retinal densitometry that there were two visual pigments in the red-green range upon the normal fovea, and only one upon the protanope's (Rushton, 1955). Now that this pigment chlorolabe has been studied by itself in the protanope (Rushton, 1963a, b, c) and a more red-sensitive pigment erythrolabe has been studied by itself in the deuteranope (Rushton, 1965a, b) we are in a position to begin the study of the mixture on the normal fovea.The red pigment is the easiest to isolate, so in this paper we shall analyse that and show that it appears to be identical with erythrolabe of the deuteranope by the four kinds of measurements that can be used-the difference spectrum, the action spectrum, the photosensivity and the regeneration rate. It will also be shown to be a visual pigment by the two kinds of measurement that can be used-the correspondence of the action spectrum with the visual spectral response of Stiles' red mechanism 7T5 and the correspondence of the time course of pigment regeneration with the dark adaptation curve of T5. METHODSThe equipment and procedure was the same as in the other investigations upon foveal pigments and has been already described in some detail (Rushton, 1963a). In brief, subjects with the left pupil dilated by homatropine were fixed in exact alignment in the densitometer by a dental impression and brow rest. They fixated upon cross-hairs in the centre of a 21' field that constituted the retinal region upon which the bleaching and measuring lights fell. Light entered the eye by Maxwellian view through the nasal half of the pupil and the reflected portion that left through the temporal half encountered a small mirror and was deflected into a photocell. By means of a lens and a stop only light coming from the direction of the central 2°of the fovea was accepted by the photocell. Light from the anterior part of the eye could therefore only be received by a double horizontal scatter and the signal was polarized in the sense that made scatter chiefly vertical. A bleaching light could be substituted for the measuring light upon the fovea by interposing a mirror. Two such mirrors mounted like the sails of a windmill were generally used for bleaching. The windmill turned at 5-10 rev/sec and flashes of bleaching light were applied at twice this frequency, shining through i of the cycle. Between the bleaching flashes the measuring light fell upon the same retinal region and by means of an electromagnetic shutter only the measuring light reached the photomultiplier cell. In this way it was possible to measure retinal reflectivity at one wave-length while bleaching with light of ayother wave-length.

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