102 o THE SENSES 



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-sensa- 

 tions. 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 mono- 

 chromatic 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 func- 

 tions, and that the hypothetical mechanism for twilight vision (the 

 rods) is functioning alone. In another condition (night-blindness, 

 or hemeralopia) 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 any- 

 thing abnormal in their vision. Thus, to quote the words of a 

 distinguished writer on this subject, himself a sufferer from the 

 deficiency: ' A naval officer purchases red breeches to match his 

 blue uniform ; a tailor repairs a black article of dress with crimson 

 cloth; a painter colours trees red, the sky pink, and human cheeks 

 blue.' The shoemaker, Harris, the discoverer of colour-blindness, 

 picked up a stocking, and was surprised to hear other people 

 describe it as a red stocking; it seemed to him only a stocking. 

 The celebrated Dalton was twenty- six years of age before he knew 

 that he was colour-blind. He matched samples of red, pink, 

 orange, and brown silk with green of different shades; blue both 

 with pink and with violet; lilac with grey. 



When the condition of vision in dichromates is tested by means of 

 the spectrum, it is found that they fall into two classes: (i) A class 

 (of green-blind) by whom the whole of the spectrum from red to yellow is 

 d 'scribed as yellow of different degrees of brightness (intensity) ; the green 



