ir. A. H. RUSHTON 711 



(.lark is obtained trom equation (2) by putting 7 = 0, hence 



Thus the great rise in A^ which occurs after light adaptation is due 

 either to a great increase in the nerve threshold AV, or to an increase 

 in //„ the "dark light" of the system. This may be settled by experi- 

 ment, as may be seen by rewriting equation (2) in the form: 



'-^„ --(£ + ') 



(■?) 



The increment threshold curve may be obtained first in the dark- 

 adapted condition (giving the Stiles curve Fig. 2 when log A^^ is 

 l^lotted against log /) . Now the measurements are repeated in a 

 light-adapted state, and if A^ alone is changed the curve of Fig. 2 

 \\ill become shifted bodily upward by an amount equal to the change 

 in log absolute threshold. But if Ijj is what alters, both scales will 

 shift equally, so that the curve of Fig. 2 will move up as much as 

 before but also equally to the right. 



Published records of this experiment are disappointingly meagre 

 and imprecise, though nearly all curves show some shift to the right. 

 For me, however, the most convincing results are some unpublished 

 experiments on my own cones. These were adapted to light so in- 

 tense that their threshold rose 1,000 times, and the increment thresh- 

 olds were nearly exactly consistent with 7^ alone changing. They 

 diverged, however, by several 100-fold from the expectation that only 

 /\V changed. And so, though further experiment is certainly needed, 

 I shall assume here that Ip alone changes in light and dark adaptation. 



We thus conclude that a bright light, when extinguished, leaves 

 the eye with an increase in the value of lu, the intrinsic noise of 

 the retina. 



II. The Photochemical Theory of Vision 



Hecht began his work, which was to have so immense an impact 

 upon the science of vision, by the study of visual responses in in- 

 vertebrates in the early 1920's, when it was fashionable to postulate 

 unspecified chemical reactions to account for observed biological re- 

 sponses. The straightforward balanced reactions postulated for the 

 withdrawal of the siphon of the clam Mya were found to simplify 

 into equation (2) , above, for the equilibrium condition under a 

 steady light I, where A^" now represents the amount of pigment 

 photolysed by the flash A-^- We have seen that equation (2) describes 



