THE VISUAL PIGMENTS 



Using these effects as the criterion of response, granit found that 

 the retinae of most animals yielded several sensitivity curves accord- 

 ing to the state of retinal adaptation and the position of the micro- 

 electrode. Thus although only one type of curve — the *scotopic 

 dominator' — could be obtained from the dark-adapted retina of a 

 given animal, the same retina, when hght-adapted generally showed 



100 — 



400 



500 

 Wavelength in mp 



600 



Fig. 1.3. Comparison between the density spectrum of human visual 



purple (continuous curve) and Crawford's (1949) spectral sensitivity 



data for the dark-adapted human eye. 



{After Crescitelli and Dartnall, 1953) 



two types — the *photopic dominator' and several 'modulators,' 

 maximal in different spectral regions, granit considered that the 

 function of the dominators, mechanisms with broad spectral sensi- 

 tivity curves, is to make 'a large number of wavelengths available for 

 vision' ; that of the modulators, mechanisms with much narrower 

 sensitivity curves, the mediation of wavelength discrimination. 

 granit's 'dominator-modulator' theory (1943, 1947) thus suggests 

 in visual terms that both the dark- and Hght-adapted retina have their 



6 



