GEORGE WALD 747 



A similar adjustment may be needed in the region of 100 per cent 

 rhodopsin. Rushton and Cohen (39) concluded on theoretical 

 grounds, ami I (onfirmed with direct measmements (42) that in the 

 human eye the bleaching of the first few per cent of rhodopsin must 

 cause a disproportionately large rise of threshold. Indeed, my meas- 

 inements seemed to show that the bleaching of the first 0.6 per cent 

 of rhodopsin raised the threshold 3300 times (42) . To be sure, that 

 was human light adaptation, and might not involve to this degree 

 the different phenomena in another animal, shown in Fig. 8. It is 

 possible also that the relationship between threshold and concentra- 

 tion is different, in the neighborhood of 100 per cent rhodopsin, for 

 light and dark adaptation; that the bleaching and regeneration of 

 rhodopsin involve a degree of hysteresis. In any case, any such exag- 

 gerated change of threshold with pigment concentration in this region 

 should bend the line of Fig. 8 so as to make it concave to the origin 

 at the upper left. 



If both these changes were pertinent, the relationship shown in 

 Fig. 8 Avould be changed from a straight line to "a flattened reversed 

 S," approximately linear only over its middle region. That may in- 

 deed be the true condition. It is significant that as the straight line 

 is drawn in Fig. 8, almost all the points to the left lie above it, while 

 almost all those to the right lie below it; a flat S-shaped curve would 

 fit the data better than the straight line that is shown. Only further 

 measurements ■will decide finally between these alternatives. 



REFERENCES 



1. Ball, S.. Collins. 1-. D., Dalvi, P. D., and Morton, R. A., Biochem. J., 45, 304 



(1949). 



2. Blanchard, J.. Pliys. Rex'., Ser. 2, 11, 81 (1918). 



3. Bliss. A. F.. Arch.' Biochem. and Biophys., 31, 197 (1951). 



4. Bouman, M. A., Documents Ophthalmol., 4, 25 (1950). 



5. Brindley. G. S.. Phxsiologx of the Retina and J'isual Pathiuay, Chap. 1, Arnokl. 



London (1960). 

 f). Brown, P. K., and Wald, C, J. Biol. Chem.. 222, 865 (1956). 



7. Campbell, F. W., and Rushton, W. A. H., J. Physiol.. 130, 131 (1955). 



8. Crescitclli, F.. in Photohiolooy, Proc. 19th Biologv ColkKpiium, Oregon State 



College, p. .30 (19.58). 



9. Denton, E. J., in J'isiial Problems of Colour. Natl. Phys. Fab. Synip. 8. H. M. 



Stationery Office, London (1958). 



10. Denton, E. J., Proc. Roy Sac. {London), B. 1.50, 78 (1959). 



11. Donner, K. O., and Granit. R.. Acta Physiol. Scand.. 17. 161 (1949). 



12. Dowling, J. E., Nature, 1960, in press. 



13. Dowling, J. E., and \Vald, C;., Proc. Xall. Acad. Sci. U. S., 44, 648 (1958). 



