Cone pigment kinetics in the protanope

In the first of these three papers (Rushton, 1963a) the technique of retinal densitometry was applied to the fovea of the protanope. It was found that there was only one photosensitive pigment present, since bleaching either with red light or with blue-green light caused an identical change; moreover, this single pigment, 'chlorolabe', was the visual pigment of the fovea, since lights of various wave-lengths which the protanope judged to be equally bright produced equal physical bleaching of chlorolabe. In Fig. 2 of that paper chlorolabe was seen to bleach in the light and regenerate in the dark in the expected manner, and in this paper we consider more closely what are the kinetics underlying these changes. The object of our study is to derive an equation free of all arbitrary constants which will enable us to predict the value of p, the fraction of total chlorolabe present in the cones, at each moment during a bleachingregeneration manoeuvre of any kind. What we actually observe is not the pigment density, D, but the setting of the measuring wedge when adjusted to balance the foveal reflexion at 540 mp against the control at 700 m,, at which wave-length chlorolabe is transparent. The change in wedge density, W, measured from the position of full bleaching is called the 'double-density' of the pigment. Now the relation between W, which is observed, and the density D, which we need to know, depends upon the amount of stray light reaching the photocell whose output forms the basis of the measurement. If in conditions offull bleaching the straylight is atimes the light passing through the cones, the general relation is