vii RETINAL EXCITATION 383 



when tonally and simultaneously stimulated arouse l.hr sensation 

 of yellow. 



On this theory typical eases of congenital colour-blindness, 

 total or partial, are readily explained by assuming that owing to 

 some arrest of development panchroinatisation, i.e. subdivision of 

 the primitive substance which oil stimulation gives rise to the 

 sensation white, is wholly or partially defective. 



Scheuck's theory explains the different visual sensibility of 

 the ditl'ereut areas of the retina. \Ve have seen that the central 

 fovea is sensitive to all colours, the intermediate region blind to 

 red and green, the periphery entirely colour-blind; also that the 

 limits of the intermediate zone which is blind to red and green are 

 not fixed, but alter according to the conditions of experiment ; on 

 increasing the size, luminous intensity, and saturation of the test- 

 object, the limits extend outwards. 



These facts, which point to a functional differentiation between 

 the central and peripheral parts of the retina in regard to colour- 

 vision, can be explained on the assumption that panchromatisation 

 is less advanced in the periphery than in the centre of the retina. 

 But as the differences are relative, not absolute, it would, says 

 Schenck, be fallacious to conclude that the more peripheral cones 

 contain only the primitive white substance, and that others in the 

 middle zone are in the first stage of panchromatisation, and others 

 again, in the centre, are completely panchromatised. 



It agrees better with the facts to assume that in the adult 

 retina each cone contains all three chromatic substances, but in 

 varying degrees of excitability, according to their relative positions, 

 so that some are more sensitive to stimulation than others ; like 

 the different sensitive layers of a photographic plate. The fact 

 that the area of peripheral colour-vision is larger when the size, 

 luminous intensity, and saturation of the colour of the test-object 

 are increased is parallel to the sensibility for white light, and 

 is explained by assuming that adjacent cones have a reciprocal 

 influence which favours the spread of excitation. The accurate 

 work of Hess (1889) and of v. Kries (1904) showed that two lights 

 of different colour, whether compound or homogeneous, which are 

 of equal intensity when they fall on the centre of the retina, are 

 also equally bright to every peripheral part of it. Moreover, other 

 conditions being equal, any light appears uniformly bright to 

 the different retinal areas, whether it is seen coloured by the 

 central part or colourless with the more peripheral portions. 



These facts, which prove uniformity of vision in the centre and 

 the periphery of the retina, are adequately explained by Scheuck's 

 theory, which assumes that each visual substance consists of two 

 parts one to receive and transmit the stimulus, the other to 

 excite the sensation. The first determines brightness, the second 

 colour ; the first explains the equal sensibility to brightness of 



