Artificial Colour-blindness to Successive Contrast. 209 



the lens and the slit. The effect produced is that of a broad, continuous- 

 spectrum, with a narrow but much brighter spectrum in the middle of 

 it. After a few seconds a black card is suddenly brought behind the 

 lens, so as to screen off the light of the focussed lamp B. A dark band 

 like a shadow instantly appears in place of the narrow bright spectrum — 

 that is to say, the effect upon the retina of light of any wave-length is to 

 blind the eye temporarily fm' light of that same toave-length. This may be^ 

 illustrated in another way. Place near the slit of the spectroscope a 



Fig. 1. 



JBunsen burner, and behind it, a few inches farther off, a lamp, and hold' 

 between the lamp and the Bunsen flame a black card. Burn some 

 calcium or strontium chloride, or common salt, or anything that gives 

 a good bright-line spectrum, in the Bunsen flame, keeping the eye 

 fixed on one of the lines. On snatching away the card and the Bunsen 

 flame a dark-line spectrum will be seen momentarily against the con- 

 tinuous spectrum of the lamp, so sharply defined that it is difficult ta 

 realise that it is merely an illusion. 



These results are of cardinal importance. They mean that the 

 green or blue subjective impression produced by a white surface when 

 the eye has been fatigued for red does not indicate that red excites 

 an after-sensation of green or blue, or renders the eye more sensitive 

 to green or blue, but that the eye has become less sensitive to 

 red. And similarly with the other colours. This point is clearly 

 brought out by Darwin. An "accidental" colour has therefore this 

 in common with an absorption spectrum — that it involves a diminution 

 of the intensity of a certain portion or portions of the spectrum. 



The line of proof is completed by the third disposition of the vari- 

 ables. 



3. After fatiguing the retina by any one colour, to observe the entire 

 spectrum. 



This is in effect a mere variant of the method described in my paper,"^ 

 and depends on the production of a very transient colour-blindness. 

 It is necessary to make special arrangements for suddenly substituting 

 a complete spectrum for a field of view illuminated by monochromatic 

 light. Among the methods I have tried, the following may be 

 mentioned : — 



* ' Phil. Trans.; B, vol. 191 (1899), p. 4. 



