117 



On Negative After-Images with Pure Spectral Colours. 

 By George J. Burch, M.A., D.Sc. Oxon., F.R.S. 



(Received October 17, 1912,— Read January 16, 1913.) 



In a paper " On Negative After-Images and Successive Contrast with Pure 

 Spectral Colours," by Mr. A. W. Porter, F.R.S., and Dr. F. W. Edridge-Green,* 

 the authors describe certain experiments, which they consider impossible of 

 explanation on either the Hering or the Young-Helmholtz theory of colour 

 vision. 



In justice to Thomas Young, it is only fair to point out a discrepancy 

 between the title of the paper and the experimental conditions therein 

 described, viz. : " 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 on a screen. A portion of the 

 retina of one eye was then fatigued by rigidly gazing at a portion of another 

 spectrum, isolated in the Edridge-Green colour-perception spectrometer. . . . 

 After the fatiguing light had been viewed for 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." 



The italics are mine. It is impossible too strongly to emphasise the fact 

 that a spectrum projected in a room, " in which there is a certain amount of 

 stray light," cannot be regarded as consisting of pure spectral colours. 



The phenomena recorded can all be explained when the stray light is 

 taken into account, and they agree perfectly with Young's theory. Moreover, 

 they are familiar in laboratory practice. Thus, in paragraph 1, the effect 

 of red light on the blue and violet, rendering these darker and bluer along 

 the line of the after-image, is easily understood if we regard these colours as 

 contaminated with white, the red element of which is removed by the fatigue. 



In paragraphs 6, 7, 8, and 9, the fatiguing ranges were orange-yellow, 

 yellow-green, blue-green, and blue as far as X 475, and the after-images are 

 said to have been purple, evidently by admixture with the violet of the stray 

 light. But in paragraph 10, with fatiguing light X445-A455, the after-image 

 was yellow-green — clearly because the violet of the stray light was cut out 

 by fatigue. 



In paragraphs 16 to 20, experiments of a more complex character are 

 described, all, however, capable of explanation in accordance with Thomas 

 Young's theory, if the stray light is taken into account. This part of the 



* ' Boy. Soc. Proc.,' 1912, B, vol. 85, p. 434. 



