THE SENSES 949 



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 described as yellow of different degrees of brightness (inten- 

 sity) ; the green appears as a pale yellow with a grey or white band 

 in its midst ; while the violet end is seen as different shades of blue. 

 (2) A class (of red-blind) whose whole spectrum, from red to green, 

 is seen as green of different intensities, the extreme red being entirely 

 invisible. The violet end is blue, as in (i), and there is a band of 

 white or grey near the blue end of the green. 



Sir John Herschell explained Dalton's peculiarity of vision on the 

 hypothesis that he only possessed two, instead of three, primary 

 sensations. 



On the Young-Helmholtz theory, the missing sensation is supposed 

 to be either red or green. At the intersection of the curves that 

 represent the violet and green sensations (Fig. 413), the red-blind 

 individual will see what he describes as white viz., the sensation 

 produced by the stimulation of the only two components he pos- 

 sesses. Similarly, at the intersection of the red and violet curves 

 the green-blind person will see what is white to him. 



Those who have attempted to explain colour-blindness on Hering's 

 theory have usually assumed that the colour-blind possess the blue- 

 yellow, but lack the green-red visual substance. So that on this 

 theory there should be no difference between red-blindness and green- 

 blindness. But v. Kries, in a study of twenty cases of congenital 

 partial colour-blindness, brings forward strong evidence that the 

 red-green blind can be divided, as regards the comparison of red 

 (lithium) and orange (sodium) light, into two sharply-separated 

 groups a result which is, so far as it goes, in favour of the Young- 

 Helmholtz theory, and against the theory of Hering. 



The observations of Burch on temporary colour-blindness pro- 

 duced by placing the eye behind a transparent coloured screen and 

 focussing a beam of strong sunlight on it, lend additional support 

 to the former theory. Thus, if a spectrum is looked at after green- 

 blindness has been induced by exposure of the eye to green light, 

 the red portion of the spectrum seems to pass into the blue, and no 

 intermediate green band is seen. If the eye is exposed to yellow 

 light it becomes temporarily blind not only for yellow, but also for 

 red and green. This is in favour of the assumption of the Young- 

 Helmholtz theory that the sensation of yellow is caused when the 

 retinal elements concerned in the production of the sensations of 

 red and green are simultaneously stimulated. It is, however, 



