948 A MANUAL OF PHYSIOLOGY 



it may be said that the retinal rim is even then relatively and, 

 under ordinary conditions, absolutely, colour-blind. This and 

 other facts have given rise to the theory (p. 935) that the rods, 

 which are alone present at the ora serrata, are concerned in 

 achromatic vision (under conditions of dark adaptation), the 

 cones in colour vision as well as in achromatic vision (under 

 daylight conditions). 



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

 menon of great 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 distinguishing 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 tone, 

 everything appearing as white, grey, or black (total colour- 

 blindness, sometimes called monochromatic vision). All colours 

 are confused by them, but differences of brightness are correctly 

 appreciated. Probably the totally colour-blind person receives 

 somewhat the same impressions from a coloured picture as the 

 normal person does from a reproduction of the same picture in 

 black-and-white. There are close resemblances between the 

 vision of the totally colour-blind eye and that of the normal 

 eye adapted by resting in the dark for twilight vision. The 

 fovea is relatively, and in some cases absolutely, insensitive to 

 light, while the peripheral portion of the retina is normal, or 

 nearly normal, in this regard. This is the foundation of the theory 

 that in total colour-blindness the cones are devoid of their normal 

 functions, and that the hypothetical mechanism for twilight 

 vision (the rods) is functioning alone. In another condition 

 (night-blindness, or hemeralopid) it is sometimes assumed that 

 the other mechanism (that of the cones) which is adapted for 

 daylight vision, and has little power of dark-adaptation, is alone 

 acting. But it cannot be said that this has been proved. 



The rest of the colour-blind are dichromatic i.e., their colour 

 reactions seem to correspond only to two of the fundamental 

 colour sensations of the normal person and their combinations, 

 in addition to white. Of the dichromates a very few confuse 

 blue with yellow. 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 be made by the colour-blind, without necessarily 

 leading them to suspect that there is anything abnormal in their 

 vision. Thus, to quote the words of a distinguished writer on 



