1 1 io VISION. 



that diminution in the sensibility for white light should involve diminu- 

 tion in sensibility for coloured light, and vice versd. 1 It is found, however, 

 that the two kinds of excitability vary independently of one another. 

 If a large coloured (red) surface be observed after prolonged fixation of a 

 black disc on a white ground, the part of the coloured field previously 

 exposed to black appears much less saturated than the rest of the 

 coloured surface. According to this theory, fatigue for white involves 

 fatigue for red, but this experiment would mean that a part of the retina 

 fatigued for red gives a better red sensation than one which has been 

 exposed to black, i.e. has not been fatigued at all. 



The influence which fatigue of the retina to one colour has on the 

 colour-tone of different parts of the spectrum, has been much discussed 

 in relation to the Helmholtz theory. Hering regards these facts as 

 irreconcilable with the theory. The yellow of the spectrum does not 

 change in colour-tone with fatigue ; it only undergoes change in satura- 

 tion, and the same is the case with blue (v. Kries, Hess, etc.). But since 

 by this theory yellow is due to predominant excitation of the red and green 

 substances, and blue to that of the green and violet substances, blue should 

 change in tone, after fatigue to yellow, and appear violet, i.e. of the two 

 processes upon which blue depends only the green has been affected (or 

 affected to any extent) by the fatigue to yellow. Again, to a retina 

 fatigued for spectral blue, violet appears red (Exner, Hess). Violet 

 light by this theory has a slight action on the red substance, and yet 

 here the action of the red substance predominates over and masks the 

 action of the violet substance which has only been partially fatigued by 

 blue light, and is being stimulated by its own special stimulus, namely, 

 violet light. 



Helmholtz 2 held that these and other similar objections only apply 

 if the fundamental colours are to be found in the spectrum, and that they 

 are met by the assumption already mentioned, that light of any wave- 

 length acts on all three substances ; the difference between the action of 

 different lights being only quantitative. Hess, 3 on the other hand, holds 

 that the changes of colour- tone after fatigue are incompatible with any 

 form of the three-colour theory. 



Helmholtz never attempted to explain contrast physiologically, and 

 was content with the psychological explanations already considered, v. 

 Kries has also regarded contrast as chiefly a psychological matter, and 

 as unimportant for a physiological theory of vision. 



Colour-blindness. — The facts of colour-blindness, together with 

 the condition of the peripheral retina, may next be considered. At 

 one time the facts of colour blindness seemed to be strongly in favour of 

 the theory, and, according to Maxwell, colour equations, in the common 

 form of the defect, were consistent with the absence of one of the three 

 normal components. In the scoterythrous group the red component 

 was supposed to be absent, while in the photerythrous group the green 

 component was wanting, and the two groups have since been commonly 

 known as red-blind and green-blind respectively. The rare cases of con- 

 fusion between yellow and blue were held to depend on absence of the 

 violet component. 



At an early stage, however, doubts were raised. The correspondence 



1 Arch. f. d. ges. Physiol., Bonn, 1888, Bd. xliii. S. 329. 

 - "Handbuch d. physiol. Optik," 2te Aufl., S. 517. 

 3 Arch. f. Ophth., 1893, Bd. xxxix. Abth. 2, S. 45. 



