THE VISUAL PIGMENTS 



exactly matched one made after bleaching through neutral filter 

 D + 0-6, this would show that the density change due to bleaching 

 had been 0-5. In this case the photographs taken through D + 0-2 

 before bleaching and through D + 0-7 after would also match, and 

 so on. 



When it was desired to measure the average retinal density changes 

 on bleaching, the matching was done by means of a photoelectric 

 densitomer. Small portions of the films were interposed between a 

 constant source and a photoelectric cell. In these measurements it 

 was immaterial whether the image of the photoreceptors was sharply 

 in focus (as in Plate 7.1) or so far out of focus as to result in a uniform 

 density over the whole film. 



When the changes in individual photoreceptors were required, the 

 image had to be in accurate focus. Print enlargements of the films 

 were made and the comparisons were done visually with the aid of a 

 low-power binocular microscope. 



MEASUREMENTS ON INDIVIDUAL PHOTORECEPTORS 



The results of measurements made on individual rods in six 

 different frogs' retinae are given in the following table. 

 The density changes on bleaching given in the table refer to 



188 



