292 COLOUR VISION 



the light causing it. Therefore in the impulse itself we have the physio- 

 logical basis of light, and in the quality of the impulse the physiological 

 basis of colour. The impulse being conveyed along the optic nerve 

 to the brain, stimulates the visual centre, causing a sensation of light, 

 and then passing on to the colour-perceiving centre, causes a sensation 

 of colour. But though the impulses vary in character according to 

 the wave-length of the light causing them, the colour-perceiving centre 

 is not able to discriminate between the character of adjacent impulses, 

 the nerve cells not being sufficiently developed for the purpose. At 

 most seven distinct colours are seen, whilst others see in proportion to 

 the development of their colour-perceiving centres, only six, five, four, 

 three, two, or one. This causes colour blindness, the person seeing 

 only two or three colours instead of the normal six, putting colours 

 together as alike which are seen by the normal-sighted to be different. 

 In the degree of colour blindness just preceding total, only the colours 

 at the extremes of the spectrum are recognised as different, the re- 

 mainder of the spectrum appearing grey 1 ." 



" It will be noticed that the theory really consists of two parts, 

 one concerned with the retina and the other with the whole retino- 

 cerebral apparatus 2 ." 



Edridge-Green holds that the visual purple is the sole visual sub- 

 stance and the essential feature in the retina which enables it to trans- 

 form light into visual impulses. He admits that visual purple is found 

 only in the rods and not in the cones, but he believes that it is liberated 

 from the rods and stimulates the cones. From entoptic observations he 

 considers that he has proved the inflow of visual purple to the fovea, 

 chiefly by way of four canals which radiate from the fovea and branch 3 . 

 He claims to have proved microscopically the presence of visual purple 

 between the cones of the fovea in the dark-adapted eyes of two 

 monkeys 4 . Edridge-Green thus supports the theory of the transference 

 of the visual purple which was first suggested by Mrs Ladd-Franklin 

 (v. p. 273). 



It follows from this theory that " the Purkinje phenomenon, the 

 variation in optical white equations by a state of light and dark adapta- 

 tion, the colourless interval for spectral lights of increasing intensity, 

 and the varying phases of the after-image" are present in the fovea. 



' It is reasonable to suppose that the visual purple which is formed 



1 Colour- Blindness and Colour- Perception, p. 318, 1909. 



2 Hunterian Lecture*, p. 11, 1911. 3 ./. of Physiol. XLI. 274, 1910. 

 4 Trans. Ophth. Soc. xxn. 300, 1902. 



