VISION 1 02 1 



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 Dal ton'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 repre- 

 sent the violet and green sensations (Fig. 428), the red-blind individual 

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

 the stimulation of the only two components he possesses. 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-blind- 

 ness. 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 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 production 

 of the sensations of red and green are simultaneously stimulated. It is, 

 however, equally difficult to reconcile some of the phenomena of colour- 

 blindness with the Young-Helmholtz theory. Anomalies and defects 

 of colour-sensation are common accompaniments of pathological lesions 

 of the visual apparatus, and can be produced by various drugs, as by 

 abuse of tobacco. But colour-blindness, in its true sense, is con- 

 genital, often hereditary; the colour-blind are ' born, not made.' And 

 although the condition cannot be cured, it is of great importance that 

 it should be recognized in the case of persons occupying positions such 

 as those of engine-drivers, railway-guards, and sailors, in which coloured 

 lights have to be distinguished. For, while it is true that the sensations 

 which red and green lights give the colour-blind are far from being 

 identical (Pole) under favourable conditions, it is precisely when the 

 conditions are unfavourable as in a fog or a snow-storm that the 

 capacity of distinguishing them becomes invaluable (Practical Ex- 

 ercises, p. 1069). 



Irradiation. The phenomenon known as irradiation was first 

 described by Kepler, who gave as an example the appearance known 

 as the 'new moon in the old moon's arms,' where the crescent of the 

 new moon seems to overlap and embrace the unilluminated portion of the 

 lunar disc. A white circle on a black ground (Fig. 432) appears, in 



