THE EFFECTS OF "FATIGUE" 115 



" In reality each colour sensation extends so much further that red 

 and blue overlap, and so do green and violet, the only colours which I 

 have not proved to do so being red and violet " (Burch). 



From the data in Clerk-Maxwell's papers and from his own observa- 

 tions Burch estimates the transition point between 



Maxwell, red and green, 583"7 ; Burch (average of 70 persons) 573 ; 



,, green and blue, 500*3 ; Burch (average of 70 persons) 500*3 ; 



„ blue-indigo = 449"8 ; 

 Burch, blue and violet = 448. 



Pure colours — - 



Maxwell— red = 630-9 ; green = 529-7 ; blue = 457-4. 



Burch (average of 70 persons) — red = 625 ; green = 525 ; blue = 471 ; violet = 415. 



Burch therefore thinks there are usually four colour sensations — 

 red, green, blue, and violet. He is unable to detect the existence of a 

 separate yellow by this method. 



It should be pointed out that these are facts independent of any 

 theory of colour vision, but it must be remembered that the fatiguing 

 lights were of great intensity and might well produce changes which 

 are quasi-pathological. 



Edridge-Green and Marshall ^ controvert some of Burch's statements, 

 especially those relating to fatigue for the D line. Their experiments 

 differ from his in that the intensity of the fatiguing light was much less. 

 They find that after exposure of the eye to the sodium flame for from three 

 to fifteen minutes and then looking at the spectrum the yellow is entirely 

 obliterated and only a faint band of orange separates the red and green. 

 If the eye is still further fatigued this also is obliterated, the red and 

 green meeting. The red looked rather more purple, the green bluish. 

 The blue and violet appeared diminished in intensity. There was no 

 shortening of the red end of the spectrum. 



It should be noted in this connection that Burch does not deny the 

 existence of a yellow sensation in some people, but states that he finds 

 no evidence of it himself. On the other hand Edridge-Green and 

 Marshall's experiments do not produce nearly so complete a fatigue of 

 the sensations — whether true yellow (according to them) or red and 

 green (according to Burch) — as in Burch's experiments. 



A. W. Porter and Edridge-Green^ have also investigated the effects 

 of fatigue with spectral lights. The eyes were fatigued for about 20 sees. 

 with monochromatic light and then fixed a spectrum of weaker intensity 



1 Tram. Ophlh. Soc. xxix. 211, 1909. 



2 Proc. Roy. Soc. Lond. B. lxxxv. 434, 1912. 



8—2 



