832 A MANUAL OF PHYSIOLOGY 



When the condition of vision in the great majority of the colour- 

 blind 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 (intensity) ; 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. 316), the red-blind 

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

 produced by the stimulation of the only two fibre-groups he possesses. 

 Similarly, at the intersection of the red and violet curves, the green- 

 blind person will see what is white to him. 



On Hering's theory 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 emphatically in favour of the Young-Helmholtz theory, and 

 against the theory of Hering. The observations of Burch on 

 temporary colour-blindness produced 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 pro- 

 duction of the sensations of red and green are simultaneously stimu- 

 lated. It is, however, equally difficult to reconcile some of the 

 phenomena of colour-blindness produced by disease (atrophy of 

 the optic nerve) or by abuse of tobacco with the Young-Helmholtz 

 theory, for in some of these cases the only colour seen in the 

 spectrum is blue, the rest is white ; and the theory does not provide 

 for the production of the sensation of white by excitation of a 

 single group of fibres with ordinary intensity of stimulation. Colour- 

 blindness, in its true sense, is always congenital, often hereditary; 

 the colour-blind are 'born, not made/ And although the con- 



