VII 



RETINAL EXCITATION 



363 



is only just perceptible, it will be found that there is at first total 

 colour-blindness, next red-green blindness, and lastly full sensi- 

 bility to colour. 



VIII. In 1866 Max Schultze founded the " duplicity theory " 

 of the functions of the rods and cones, the end-organs of the 

 optic nerve, by correlating the decline of colour sensibility from 

 the centre to the periphery of the retina, with the diminution of 

 cones and increase of rods between the fovea and the ora serrata. 



160 



FIG. 174. Visual field for white and the four principal colours, from right eye of a young physio- 

 logist, obtained with a test object 1 sq. in. under good illumination. (Baglioni.) The peri- 

 pheral achromatic field is outlined in black, the coloured fields for yellow, blue, red, and 

 green in the corresponding colours. 



He concluded that the cones, which are the most highly differenti- 

 ated elements of the retina, subserve light and colour perceptions, 

 while the rods are concerned with the perceptions of light, but 

 are incapable of initiating colour sensations. 



In support of his hypothesis he pointed out the absence of 

 cones in the retina of certain nocturnal animals (bats, moles, 

 hedgehogs, and certain rodents) ; their relative paucity in some 

 of the night birds, as compared with the mammalian retina in 

 general, the cones in the day-birds being much more abundant 



