118 On Negative After-Images with Pure Spectral Colours. 



paper ends with the words : " ISTo matter what portion of the spectrum was 

 selected, the after-image, where it crossed the spectral band, was seen as a 

 grey square." That alone is sufficient to demonstrate the presence of stray 

 light. 



Under the head of Conclusions, the authors state that " the negative image 

 is much darker, more difficult to produce, and more evanescent in the 

 absence of all external light as when black velvet and the hands are placed 

 over the eyes. It is obvious, therefore, that external light has an influence 

 on negative after-images." I have used almost these identical words in 

 lecturing on this subject any time these 15 years. But I have quoted 

 them from the papers of Eobert Waring Darwin, which are printed in the 

 ' Philosophical Transactions ' for 1786, and were undoubtedly made use of by 

 Young in formulating his theory. 



It is difficult to understand how anyone can expect to find acceptance for 

 his bare statement, that " it is impossible to explain these facts on the 

 Young-Helmholtz theory of colour vision." 



I have described in my paper " On the Eelation of Artificial Colour- 

 Blindness to Successive Contrast "* various methods of observing the 

 phenomena of successive contrast with really pure spectral colours, using 

 stimuli no stronger than those employed by Mr. Porter and Dr. Edridge-Green. 

 My results are different from theirs, and are in all respects quite in accordance 

 with the theory expounded by Thomas Young. 



It is a matter of everyday demonstration in the laboratory that, using 

 moderate stimuli, with persons of normal colour sensation, yellow does change 

 to green after fatigue to red, and to red after fatigue to green. And I am 

 bound also to note that persons whose green sensation is weak fail to see this 

 change in the colour of yellow after fatigue. 



* ' Roy. Soc. Proc.,' 1900, B, vol. 66, p. 206. 



