420 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



new formed material also falls to pieces and the correlative 

 sensation is greatly enhanced. All the simple phenomena of after- 

 images are adequately described, but a detailed examination yields 

 less promising results. 



V. Kries found that the responsiveness of the eye to mono- 

 chromatic light was markedly altered by previous exposure to 

 white. He found that the stimulus value of the red he employed 

 was diminished in the ratio of about one to four by previous re- 

 tuning with white. 1 How can this come about if the black-white 

 substance is independent of the red-green substance ? 



Bering's own views on this matter deserve careful study.( 28 ) He 

 objects that, in v. Kries' experiments, the colour used as a reactor 

 (180 blue, 180 white on a disc) was not sufficiently saturated, 

 and brings forward the following experiment : 



Two discs are arranged, A and B. A consists of a black centre 

 surrounded by a white ring, B of a centre composed of 120 blue 

 and 240 black, encircled by a ring containing 356 blue and 4 

 white. A point upon the internal margin of the white ring in A 

 is fixated for a given time, and the experimenter then turns his 

 eye to an exactly corresponding point in B. Hering always found 

 that the outer ring in B, under these conditions, looked more 

 saturated than the centre. 



As the term saturated is not used by Hering in the physical 

 sense, the exact bearing of his objection to v. Kries' experiment 

 is not apparent. Presumably what is meant is that the " blue- 

 ness " of such a disc was not distinctly separable from its 

 " whiteness," with the result that the admitted change in the 

 latter on retuning with white was mistaken for a change in the 

 former. If this objection is admissible, it is, however, equally 

 fair to retort that in Hering's counter-experiment the difference 

 between the amounts of blue in reacting and comparison fields 

 was so great that an enormous diminution in responsiveness over 

 the retuned area would be necessary for the production of a good 

 match between the centre and periphery of B. Put briefly, the con- 

 tention is that after white retuning, a reacting blue and white can 

 be made to match a pure blue in brightness by adding more white ; 

 adding more blue will always make the reactor too saturated. 

 Hering's experiment does not seem to prove more than that a 



1 For an account of v. Kries' work and some more recent observations of hi 

 pupils, see Nagel's Handb., Bd. iii., p. 210, &c. 



