STUDIES IN SPECIAL SENSE PHYSIOLOGY 421 



diminution of about 66 per cent, in apparent intensity cannot 

 be attained by retiming with the white he employed. That the 

 apparent brightness IMS correspondingly reduced is indeed favour- 

 able to his hypothesis, but it is to be noted that we have under 

 these conditions the difficulties of specific brightness comparison 

 in their most acute form. 



The discussion of conflicting experimental methods is, how- 

 ever, always a thankless and mostly a dull task ; the reader's special 

 attention is accordingly directed to the following remarkable 

 passage with which Hering concludes his memoir : 



" To the change of state experienced by an element of the 

 somatic visual field when acted upon by, e.g. blue light, with which 

 the blue sensation is associated, the whole somatic visual field reacts 

 bv a change in the opposite direction which corresponds to the 

 oppositely coloured or yellow sensation, and any light that now 

 falls on the retina acts, in consequence of this chromatic retuning 

 of the visual field, as if its yellow valency were increased and its 

 blue valency correspondingly diminished. This retuning is maximal 

 in the immediate vicinity of the element acted upon by the blue 

 light, and diminishes with its distance from the same. ... A 

 white light falling on the neighbourhood of the region which has 

 been stimulated with blue seems therefore more or less yellowish, 

 but a white light which falls together with blue on the spot that 

 has been stimulated with blue, seeing that it behaves as a more 

 or less yellow-valent light, neutralises the blue valency of the blue 

 light so much the more, the greater be the quantity of it mixed 

 with the latter. This explains the striking fact "that the chromatic 

 quality of a saturated colour is so extremely quickly extinguished 

 by increasing the amount of white mixed with it. . . ." 



" When v. Kries, therefore, supposed that, according to the 

 theory of opposite colours (Gegenfarben), the same result would be 

 obtained from a fatigued and an unfatigued area if the same 

 quantity of blue light were allowed to fall on both, but in addition 

 on the fatigued area a suitably chosen quantity of white light, he 

 was in error. In such a case, the blue valency of the blue light 

 at the unfatigued area is unaltered since no other light is mixed 

 with it ; over the fatigued area the blue valency of the blue light 

 is partly neutralised by the admixture of white. Accordingly the 

 blue at this latter area must appear less saturated than at the 

 former. In fact a transitory equality in brightness and saturation 



