204 COLOUR VISION 



rods as the more primitive type of visual neuroepithelium, as we are 

 probably justified in doing, the persistence of recognisable rod attributes 

 in the cones, even if modified, differentiated, and rendered more complex, 

 might well be expected. Apart therefore from the difficulties of isolating 

 the physiological results of excitation of the rods from those of excita- 

 tion of the cones it may be anticipated that the latter cells will retain 

 some measure of the functions which are in the highest degree charac- 

 teristic of their prototypes. Hence, if it should ever be conclusively 

 proved that the rod-like foveal cones of the human eye possess some 

 trace of visual purple and are endowed with some slight degree of light- 

 adaptation it would not be surprising ; neither, on the other hand, 

 would it militate seriously against the view that the rods and cones 

 have become essentially diverse in function. 



Beyond the rod-free central area the cones diminish continuously 

 in number and the rods correspondingly augment in passing towards the 

 periphery in all directions. 



The chief characteristic of central vision in the photopic condition 

 is its great acuity as compared with peripheral vision. This acuity is 

 most marked in the form sense, and the rapid diminution in " visual 

 acuity " in passing from the centre towards the periphery is strong 

 evidence in favour of the view that the cones are the essential retinal 

 end-organs concerned in the discrimination of form. A striking feature 

 of the vision of lower animals not endowed with a specially differentiated 

 fovea is the remarkable acuteness in the perception of the movement 

 of objects. This feature is also prominent in human peripheral vision, 

 though it is certainly equally, and perhaps more highly, developed in 

 foveal vision. Thus Ruppert^ found that while visual acuity and 

 ability to perceive movements both diminish in passing from the 

 centre towards the periphery the former diminishes more rapidly than 

 the latter. On teleological grounds the perception of movement, a 

 function specially associated with the light sense as opposed to the 

 form sense, must be regarded as primitive ; and we have here, if such 

 be needed, an example of the persistence of this primitive attribute in 

 the more highly differentiated cones. 



When the field of vision of the photopic eye is further investigated 

 it is found that the perception of colours requires an increasingly intense 

 stimulus in passing from the point of fixation towards the periphery. 

 The complicated details of peripheral colour vision have already been 



^ Ztsch. f. Sinnesphysiol. xlh. 409, 1908 ; cf. Easier, Munch, med. Woch. p. 1904, 

 1906. 



