W. A. H. RUSHTON 713 



of this relation was never stated, nor does any evidence appear to 

 have been offered to justify the faith of the hundreds who obtained 

 dark adaptation curves and interpreted them as measuring in some 

 unspecified manner the regeneration of visual pigments. Indeed, it 

 is hard to see how the relation between the dark adaptation and the 

 pigment levels in man could be investigated before a method became 

 available for measuring pigments in the living eye. We have now 

 developed such a method and have begun to investigate the relation, 

 and it is satisfactory to be able to state that the faith of former 

 workers was not misplaced, for dark adaptation turns out to be in- 

 timately related to the concentration of the visual pigment. What 

 the relation is will appear in the sequel, but it is a pleasure at once 

 to pay tribute to the brilliant intuition of Wald, who divined a 

 formulation (Wald, Brown, and Smith, 14) that is justified by our 

 measurements though contradicted by his own (13) : Wald's con- 

 tribution to this symposium, ho^vever, includes results which agree well 

 with ours. 



(a) Measuretnent of Visual Pigments in Man 



The technique of retinal densitometry has been described in de- 

 tail elsewhere (Rushton 7, 8) . Here only the bare principle will be 

 mentioned. Light from the headlights of a car reflected back from 

 a cat's eye has passed twice through the cat's retina, and thus will 

 have suffered absorption by the visual pigments there. When these 

 are photolysed they become transparent (bleached) , and the reflected 

 light will be less absorbed. If the intensity of reflected light is 

 measured on two occasions, and if the only change which has occurred 

 between them is that the visual pigment present is more or less 

 in amount on the second occasion, then the change in pigment density 

 can be measured by the change in the logarithm of the reflected light. 

 The human eye reflects only about 1/20,000 of the in-going light, and 

 various precautions must be taken on account of stray light, noise, 

 and other unwanted fluctuations. 



In practice, cone pigments may be measured upon the fovea, rod 

 pigments 15° away, with a procedure which takes 5 to 7 seconds, 

 bleaches less than 1 per cent, and gives a value for the fraction of 

 total pigment present Avhich is accurate to about 0.05. The limit 

 of the method is set by the steadiness with which the subject can 

 maintain the position of eye and head over the long periods which 

 may be involved in a whole investigation. 



