358 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



Burch, therefore, while confirming the statement that adapta- 

 tion increases the stimulus value of blue -violet relatively to that 

 of red, inferred that the achromatic threshold had no existence, 

 and that the positive results obtained by other workers were due 

 to a compounding of the immediate stimulus with the effects of 

 previous exposure to light i.e. after-images. By careful observa- 

 tions in a dark room he ascertained that diffused luminous effects 

 his " dazzle tints " persist for as long as two and a half hours 

 after entering the dark room, and that when these had disappeared 

 no achromatic interval existed for his own eye. 



This evidence is sufficiently strong to render the existence of 

 an absolute as distinct from a specific threshold for coloured light 

 exceedingly doubtful in the case of foveal vision. On the other 

 hand, it is not clear from Burch's paper that special attention was, 

 or could be, paid to the behaviour of the periphery ; so that we are 

 not entitled to conclude from it that an achromatic interval does 

 not exist when the peripheral retina is stimulated ; indeed the 

 careful researches of Tschermak and others, who worked with pro- 

 longed dark adaptation ( 2 ), are opposed to such a belief. 



Burch's results merely, I think, emphasise the view I am en- 

 deavouring to develop that central and peripheral vision differ in 

 kind and degree. His work does not appear to be fundamentally 

 new, except in experimental technique, for Parinaud in 1898 

 remarked in his most suggestive and valuable book : "At the 

 fovea, on the contrary, a simple light of sufficient purity is per- 

 ceived primarily as a colour, whatever be the intensity of the light, 

 or whether the retina be or be not adapted " ( 10 , p. 51). 



Without therefore going the length of asserting that an 

 achromatic interval depends in some vague way on previous stimu- 

 lation, we must be very cautious in interpreting results, such as 

 Stegmann's, obtained by this method. 



So far, then, direct experiment seems to have established the 

 following points : 



(1) The peripheral regions of the retina are relatively more 

 sensitive than the fovea to light of moderate or short wave length. 



(2) Adaptation to darkness is characterised by an increase in 

 responsiveness to short waved light, and this change is predomi- 

 nantly, if not entirely, extra-foveal. 



The following tables illustrate these statements. The intensity 

 values are arbitrary, the measurement of eccentricity is in terms 

 of the angle subtended at the nodal point. 



