8 3 o 



A MANUAL OF PHYSIOLOGY 



shown by Aubert and others that monochromatic light of considerable 

 intensity can be perceived over the whole retina, yet it may be said 

 that the retinal rim is even then relatively and, under ordinary condi- 

 tions, absolutely colour- 

 blind. This and other 

 facts have given rise to 

 the theory that the rods, 

 which are alone present 

 at the ora serrata, have 

 for their function the 

 mere perception of lu- 

 minous impressions as 

 such, without any dis- 

 tinction of quality or of 

 colour. The cones are 

 supposed on this theory 

 to be more highly de- 

 veloped than the rods, 

 their function being con- 

 nected especially with 

 the perception of colour. 

 But although there are 

 certain histological facts 

 which favour the view 

 that the cones are a 

 more highly - developed 

 form of the rods, it can- 

 not be considered as 

 proved that animals whose retinae are devoid of cones are totally colour- 

 blind. And, indeed, the observations of Hess and Hering on total 

 colour-blindness in man show that the assumption of v. Kries and 

 Konig, that in this condition the cones are either entirely insensitive 

 or replaced by rods, is unfounded. 



This brings us to the subject of colour-blindness, a pheno- 

 menon of the greatest interest in its theoretical as well as 

 in its practical bearings. 



Colour-blindness. A considerable number of persons 

 (about 4 per cent, of all males, but only one-tenth of this 

 proportion of females) are deficient in the power of distin- 

 guishing between certain colours. They are said to be 

 colour-blind ; but the term must not be taken to signify that 

 they are absolutely devoid of colour-sensations. A very 

 small minority of the colour-blind appear to have but one 

 sensation of colour ; a few confuse green with blue ; the 

 great majority are unable to distinguish between red and 

 green. The condition will be most easily understood by 

 considering some of the extraordinary mistakes which may 



FIG. 318. PRIESTLEY SMITH'S PERIMETER. 



K, rest for chin ; O, position of eye ; Ob, object, 

 white or coloured, which slides on the graduated arc 

 B ; f, point fixed by the eye. 



