STUDIES IN SPECIAL SENSE PHYSIOLOGY 371 



justified in assigning to the periphery a predominating share in 

 the work of dark adaptation. 



We have also seen that these adaptive changes consist in a 

 greatly increased responsiveness to light of short wave length, 

 such appearing more intense than under " light " conditions. 



We have thus to account theoretically for a localised change in 

 responsiveness with respect to certain forms of stimulation. 



The difference in histological structure between the fovea 

 centralis and the surrounding area caused Schultze forty years 

 a g ( 24 ) to suggest a functional separation which he supported 

 on evidence drawn from comparative anatomy. In his time, 

 however, our experimental knowledge of adaptive changes was 

 little advanced and his conception went unheeded. The first de- 

 tailed investigation was due to H. Parinaud ( 26 ), who has developed 

 a theory of adaptation in a series of memoirs dating from 1881. 

 Similar views have been carefully elaborated by Professor J. v. 

 Kries of Freiburg and his colleagues and pupils in a large number 

 of well-planned researches. 



Essentially the theories of Parinaud and v. Kries may be 

 summarised quite simply. Two distinct visual mechanisms exist : 

 one, subserving both chromatic and achromatic responsiveness, and 

 represented in the retina by the cones ; the other dealing with 

 achromatic sensations alone, represented by the rods and visual 

 purple. The former mechanism is alone active in bright daylight 

 and is unaffected by resting in the dark ; the latter is brought into 

 play by shielding the eye from stimulation, being the sole or chief 

 agency of twilight vision ; it is characterised by special responsive- 

 ness to ethereal vibrations of short wave length. In view of the 

 double nature of the mechanisms postulated, the theory has been 

 christened the Duplicity Theory (Duplizitatstheorie). Let us see 

 how far the hypothesis covers the experimental observations I 

 have enumerated. 



If the theory be true, we should expect (1) spectral maximum 

 brightness to change in weak light in favour of the violet end ; 

 (2) this change not to occur for images formed at the fovea 

 centralis ; (3) no achromatic threshold (vid. sup.) to be obtained 

 for any light at the fovea or for red light anywhere. 



We have seen that each of these deductions is supported by 

 good experimental evidence. 



Again, have we any forms of vision in which, apparently, the 



