COLOUR VISION AND COLOUR-VISION THEORIES 481 



cerebral apparatus be fatigued by light of a certain wave-length, 

 a negative after-image will appear through simultaneous contrast. 

 If one portion of the visual area be less sensitive for impulses 

 caused by light of a certain wave-length (for instance, red), and 

 the adjoining areas be stimulated by impulses corresponding to 

 light of all wave-lengths, the image corresponding to the fatigued 

 area will be relatively blue-green to the images corresponding 

 to surrounding areas. This explanation on the theory of colour 

 vision given is in accordance with the other facts of simultaneous 

 contrast. 



It is impossible to explain the facts of negative after-images 

 and successive contrast on the Hering and Young-Helmholtz 

 theories of colour vision. 



The complementary to the exciting light is never strengthened 

 in the spectrum on the screen by the after-image, as it should 

 be according to the Hering theory. When a negative after- 

 image has been formed in an absolutely dark room it becomes 

 increasingly difficult to produce this after-image on the second, 

 third, fourth, and subsequent attempts. The opposite should be 

 the result on the Hering theory. The stability of the after- 

 image is remarkable, it does not change colour or oscillate, and 

 is not surrounded by the primary colour as it should be accord- 

 ing to this theory. 



The effect of fatiguing the eye with a monochromatic region 

 produces a uniform grey band across this region. On the 

 Young-Helmholtz theory this should vary in colour and lumin- 

 osity across its breadth. On this theory the after-image should 

 change colour on fading, because of the varying amount of 

 fatigue of the hypothetical colour sensations. This is not 

 the case. Regions like violet, after fatigue to red, should be 

 very little affected, but they are the most affected. The fatiguing 

 light should chiefly affect the region used for the fatigue. This 

 is not the case. An after-image should not be seen in the absence 

 of all external light. 



IX. Peripheral Colour Vision (28) 



The erroneous statement is continually made that the peri- 

 phery of the retina is colour-blind. If red light of sufficient 

 intensity be employed it can be recognised as the same red to 

 the extreme periphery of the field of vision. This is exactly 

 what we should expect on the theory given, the less developed 

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