384 On Colour- Sensation. 



we have the two linear equations between five colour-sensa- 

 tions which are required to satisfy the laws which Maxwell 

 and Helmholtz established. Therefore the hypothesis of a fifth, 

 white, colour-sense is admissible. 



That the eye does perceive white separately from any other 

 colour is rendered at least probable by considering some par- 

 ticular cases in which this seems to occur. 



(1) When an object is viewed indirectly, so that the image 

 falls upon a part of the retina at a sufficient distance from the 

 yellow spot, it will appear white or grey, whatever its actual 

 colour may be (von Kries, pp. 91-95). 



(2) If the angular dimensions of a coloured object be dimi- 

 nished, it will ultimately appear white or grey (von Kries, 

 pp. 87, 94). 



The more probable explanation in both these cases seems to 

 be that the other colour-senses are no longer affected by the 

 object, and only the white colour-sense remains affected 

 by it. 



(3) Every colour when intensely lighted up appears white 

 (von Kries, p. 81). A not improbable explanation of this 

 seems to be that the other colour-sens?s are only capable of 

 being affected by light to a limited extent as compared with the 

 white colour-sense. 



(4) In cases of atrophy of the optic nerve the perceptions of 

 different colours are gradually lost, until at length every colour 

 appears grey (von Kries, p 154). 



(5) There are also cases of total colour-blindness from birth, 

 when every thing appears of the same colour with only dif- 

 ferent degrees of light and darkness. When this affects both 

 eyes completely, it is of course impossible to predicate with 

 absolute certainty what colour is perceived. But Becker 

 (Graf's ArcJiiv, vol. xxv.) describes a case where only one 

 eye was so affected, the other having normal vision; and 

 I have seen another case described in which one half of each eye 

 was completely colour-blind, the other half being normal. In 

 each of these cases the colour-blind vision was of white. This 

 white vision must have been arrived at either through the 

 other colour-senses having been lost, leaving a white colour- 

 sense behind, or through their having been modified into 

 white. 



