928 
Prelinunary Note on a Method of Measuring Cotour-sensations 
by Internuttent Inght, with Description of an Unfinished 
Apparatus for the Purpose. 
By GrorGE J. Burcu, M.A., D.Sc. (Oxon.), F.R.S. 
(Received February 7,—Read March 30, 1911.) 
When I began in 1895 to apply the methods of artificial colour-blindness 
to test the colour-sensations of other people, I cast about in various 
directions for means of intensifying the effect while reducing the brilhancy 
of the light used to produce the necessary fatigue. 
The results obtained, though not immediately available for my purpose, 
were in several cases of sufficient interest to warrant their publication. 
Successive Contrast—In my paper on the “ Relation of Artificial Colour- 
blindness to Successive Contrast,” I described* a device whereby after 
fatiguing the retina with any one colour a complete spectrum might be 
suddenly substituted for it, thus producing “a very transient colour- 
blindness” which may be for the moment even more nearly complete than 
that described in my paperft as produced by the most intense light. For 
practical testing, however, this momentary effect is useless, requiring a 
trained eye to locate the extent of it. It forms a very good experiment in 
a practical course of physiological optics. 
Dark-adaptation.—I found that in the dark-adapted eye the lhght of an 
ordinary lamp is strong enough to produce artificial colour-blindness to light 
of less intensity.[ But there are many reasons why dark-adaptation is 
undesirable as a necessary condition of any colour test. Among others are 
the time it takes and the insuperable difficulty of the curious condition’ of 
nervousness it induces in many men, particularly among those whose colour- 
sensations are abnormal. I have, however, found it very advantageous to 
apply the spectroscopic test in a subdued light. 
Simultaneous Contrast—An attempt in 1897 to utilise the phenomena of 
simultaneous contrast in the examination of colour-sensations proved unsuc- 
cessful, the effect being much less striking than I had anticipated. Buta 
modification of the arrangement affords an excellent method of demon- 
strating and explaining the phenomena. | 
* * Roy. Soc. Proc.,’ vol. 66, pp. 210, 211. 
+ ‘Phil. Trans.,’ B, 1899, vol. 191, pp. 1—32. 
1 * Roy: Soc: ebroce; avol1oG6, p. 21: 
§ ‘Roy. Soc. Proc.,’ vol. 67, p. 226. 
