GEORC.E WALD 711 



entire chain ol processes that constitntes the visual system waits 

 npon and hangs uj)on the initial events that govern excitation by light 

 in the recej)tors, and in tlic fnst instance upon the absorption of 

 liglu by a visual j^ignient. 



I should like to defend the thesis that the visual sensitivity is 

 governed primarily by the capacity of the visual pigments to absorb 

 light, a capacity compounded from their absorption spectra and 

 their concentrations. This is a view closely associated with the work 

 of my former teacher, Selig Hecht, who developed and warmly sup- 

 ported it during his lifetime. I do not defend it on that account, 

 but because I believe it to be correct. It is also peculiarly important, 

 for it takes us from wholly external dealings with a closed organism 

 — the input of stimuli, the output of responses — to what goes on 

 inside, to the properties and reactions of its substances. 



I have develojDed this view in some detail elsewhere, and should 

 like only to review very briefly two aspects of it here: (a) that the 

 spectral sensitivity of vision derives simply and quantitatively from the 

 absorption spectra of the visual pigments; and (b) that the visual sen- 

 sitivity and its changes in light and dark adaptation are a simple 

 function of visual pigment concentration. 



Spectral sensitivities and absorption spectra. In comparing these 

 quantities it is best to measure the absorption spectra of the visual 

 pigments in the retina itself or in suspensions of rods or cones, since 

 under these conditions the spectra differ in characteristic ways from 

 those measured in solution (9, 10, 50) . This is a significant distinc- 

 tion, for it involves the recognition that in the visual receptors the 

 pigments are oriented in highly organized structures that are quasi- 

 crystalline, approximating the solid state. It must be recognized also 

 that the spectral sensitivity wath which we are concerned is that of 

 the naked receptors, undistorted by whatever colored ocular structures 

 may lie in front of or behind them, to screen or reflect back upon 

 them. 



When these considerations have been adequately resolved, the spec- 

 tral sensitivities of the receptors appear to be virtually identical with 

 the absorption spectra of their visual pigments. Some of the most 

 satisfactory of such comparisons involve Granit's microelectrode meas- 

 urements of rod and cone sensitivity in a variety of animals, as 

 shown in Figs. 5 and 6. 



It is clear from such measurements that the spectral sensitivity 

 of vision, rod and cone, whether based upon vitamin A, or Ao, 

 reflects simply and directly the absorption spectra of the associated 



