402 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



spectrum. The neutral point is in the yellow, between 566 and 

 570 ,u/u- 



At the risk of being tedious, I must emphasise the fact that the 

 truth or falsehood of the foregoing account depends entirely upon 

 the trustworthiness of the various observations. None of the 

 statements rely on any hypothesis as to the mode of action of 

 the visual elements. 



SECTION IV. AFTER-IMAGES 



Considerations of space make it impossible to discuss, even 

 in outline, the numerous interesting observations which have 

 been made upon visual after-images. I will merely, for the 

 purpose of reminding the reader of facts with which he 

 is doubtless familiar, tabulate the main results of modern 

 work. 



(1) The distinction between " positive " and " negative " after- 

 images is not absolute but relative, depending on the nature of the 

 reacting stimulus. 



(2) An image -producing or " retiming " stimulus changes the 

 stimulus value of a " reacting " (subsequently applied) light, but 

 only in such a way that the sensation response following exposure 

 to the " reacting " light is increased or diminished quantitatively. 

 Colour equations do not lose their validity. 



(3) The latter statement is true in the case of foveal vision, but 

 not for peripheral stimulation. 



(4) We do not know the time relations of the retuning process, 

 nor whether it proceeds uniformly or in pulses. 1 



(11) Piper, Zeitsch. f. Psy. u. Phys. d. Sinnesorg., vol. 38, p. 155, 1905 (Patho- 

 logical). 



(12) Levy, Arch. f. Opthalmologie, vol. 62, 1906, p. 464. 



(13) Collins und Nagel, Zeitsc. f. Psy. u. Phys. d. Sinnesorg., vol. 41, 2nd part, 

 p, 74, 1906 (Pathological). 



(14) Sclwnck, Pfl. Arch., vol. 118, p. 161, 1907. (This memoir is mainly theo- 

 retical ; it describes a case, however.) 



1 For full information, in addition to the treatises of Helmholtz and v. Kries, 

 already frequently cited, see v. Tschennak, Vber das Verlitiltnis von Gegenfarbe, 

 Kompensationsfarbc und Kontrastfarbe, Pfl. Arch., 1907, vol. 117, p. 473. G. J. 

 Burch, Proc. Roy. Soc. (1900), vol. 66, p. 204. Some a?nusing observations by 

 Goethe will be found in his Farbenlchrc (Eastlake's translation, p. 22). 



