Artificial Golotir-hlindness to Successive Contrast. 20T 



appropriated to it. The phenomena of negative and positive after- 

 images were known — it was known that certain colours are altered in 

 hue to an eye previously exposed to coloured light, and that experi- 

 ments on this subject should be made with the spectrum rather than 

 with pigments. 



The following papers also are of special interest in connection with 

 the present communication. 



Brewster,* by looking at the spectrum through a coloured medium,, 

 was able to trace the green as far as C. His mode of explaining the 

 phenomenon led to a controversy, in which the true merit of the 

 observation was lost sight of. A momentary colour-blindness is, in 

 reality, as was shown by Hunt, produced by the contrast of adjacent 

 parts of the spectrum differing greatly in brightness. 



Piazzi Smyth,! working with a very long spectrum, observed the 

 boundaries of the colours to change when any alteration was made in 

 the intensity of the illumination. 



The earliest account of a systematic investigation of the effect of 

 retinal fatigue on the colours of the spectrum is in a paper of John- 

 Aitken.J Similar observations were made by Edmund Hunt,§ who 

 also describes the appearance of the spectrum when observed through 

 certain coloured media, after the manner of Brewster. Coloured 

 figures of the results are given. Both these authors used light much 

 less intense than that employed by me, and did not obtain the full 

 effect. I was not aware of their work until after my own paper had 

 been read, and therefore take this opportunity of calling attention to 

 it. To these may be added a paper by Hess|| on the Alterations of the 

 spectral colours by retinal fatigue. 



n. — Experimental Investigation of the Phenomena of Successive Contrast. 



Successive contrast is an effect of two stimuli — a primary stimulus 

 by which the retina is fatigued, and a secondary stimulus, the effect of 

 which is modified in consequence of the first. The colour-sensations 

 excited may be reduced to four at the most. To arrive at the funda- 

 mental laws of contrast, we may vary the conditions in the following^ 

 manner : — 



Let the first stimulus excite a single colour-sensation, taking each in 

 turn, or separated from the rest as in the spectrum. Four cases- 

 arise : 



(1.) The second stimulus may excite all the sensations, i.e., it may 

 consist of white light. 



* *Edin. Trans.,' vol. 12 (1834), p. 132. 



t ' Eoy. Soc. Edin. Trans.,' toI 28 (1879), p. 792. 



X " Colour and Colour-Sensation," * Koy. Scot. Soc. of Arts Proc.,' 1871-72^ 

 § Hunt, ' Colour Vision,' Glasgow, 1892. 



y Graefe's * Archiv fiir Ophtlialmologie,' 36, abth. 1, pp. 1—32. 



