434 



Negative After-images and Successive Contrast with Pure 

 Spectral Colours. 

 By A. W. Porter, B.Sc., F.R.S., Fellow of University of London, University 

 College, and F. W. Edridge- Green, M.D., F.B.C.S., Beit Memorial 

 Besearch Fellow. 



(Received May 28 —Read June 27, 1912.) 



The object of the research was to ascertain the appearance of pure 

 spectral colours after the eye had been fatigued by light of a known wave- 

 length or situated in a portion of the spectrum between two known wave- 

 lengths. 



The method adopted was as follows : — In a dark room, in which, however, 

 there was a certain amount of stray light, a horizontal spectrum as pure as 

 possible was projected upon a screen. A portion of the retina of one eye 

 was then fatigued by rigidly gazing at a portion of another horizontal 

 spectrum which was isolated in the Edridge-Green colour-perception spectro- 

 meter.* The eyes were kept in a vertical position, that is, one over the 

 other, so that the long axis of the after-image would be at right angles to 

 the i individual colours of the spectrum when the eye resumed its normal 

 position. After the fatiguing light had been viewed for a period of about 

 20 seconds the eye was turned to the screen so that the after-image formed 

 a band' running right across the spectrum on the screen and occupying its 

 centre. By this means any change either in colour or luminosity in the 

 portion occupied by the after-image could be readily detected on account of 

 the'comparison with the colours of the unaltered spectrum seen above and 

 below (see Diagram). 



V B G Y O K 



The dotted line shows the projected after-image on the screen spectrum. 

 The fatiguing I light was much stronger than the reacting light. 



1. The eye was first fatigued with pure red light occupying the region 

 A, 654-X 675. When the eye was turned to the spectrum on the screen the 

 following changes were noted in the region occupied by the after-image : the 

 extreme red was slightly diminished ; there was no perceptible action on the 

 * ' Eoy. Soc. Proc.,' 1910, B, vol. 82, p. 458. 



