EYE, DISEASES OF. 283 



five per cent. In America it is said to be less common. Women are rarely colour- 

 blind. It is frequently hereditary, and as a niie remains unaltered throughout life. 

 It is said that iiit.-nuai-ria^'s fax our its occurrence, but really little or nothing is 

 known about its cause. In exceptional cases it comes on in adult life, possibly from 

 )f the eyes or the constant strain of looking at colours. After a railway 

 accident from not distinguishing the red light, the engine-driver confessed that for 

 some time he had been losing the power of recognising colours, and so sensible was 

 he of his deficiency in this respect that he was on the point of resigning his post 

 when the disaster occurred. The necessity of testing for colour the eyes of porters, 

 engine-driven, signal-men, navigating-lieutenants, and others employed on railways 

 or at sea is obvious. As a matter of fact, most railway companies do subject their 

 officials to a colour test, but the examination rarely extends much beyond the exhi- 

 bition of green, white, and red coloured lamps, a method which is insufficient to test 

 thoM> cases in which there is not absolute colour-blindness, but only impairment of 

 the sense of vision in this respect. A man may be able to discriminate between red, 

 blue, and green light transmitted through glasses of these colours, but may confound 

 pink with green worsteds ; and it is known that certain conditions of the atmosphere 

 may give to a red light just that kind of tint which would confuse such a man, and 

 would justify his rejection as a driver on a railway. The best way of testing the 

 sight for colour is to take a large number of pieces of worsted, variously coloured, and 

 direct the subject of examination to sort them. A single error would settle he 

 point ; the tints most likely to be confounded are pinks and light greens. It must 

 be remembered, too, that the colour-blind can distinguish reds and greens better by 

 an artificial light than by daylight. Care should be taken in testing for colour- 

 blindness, that mere ignorance of the names of colours is not mistaken for colour- 

 blindness, and thus a child be proscribed a business in which he might have 

 succeeded. 



For this complaint there is no cure and hardly any palliative remedy. It is said 

 that if a colour-blind person would wear a pair of spectacles with one eye red and 

 the other green, he might in time be enabled to form a judgment of red and green 

 things intuitively. 



Snow Blindness is a temporary loss of sight, caused by looking at the dazzling 

 whiteness of the snow. A similar condition may result from looking at an intense 

 artificial light. Lighthouse keepers, after trimming their lamps at night, are often 

 for some minutes absolutely blinded, and do not completely recover for many hours. 

 The only thing is to wear dark blue spectacles, so as to cut off the glare of the light. 



Night Blindness, or impairment of sight, varying from slight dimness to almost 

 complete darkness after the sun has gone down, occurs most frequently among sailors 

 and others who have spent much of their time in the tropics. The glare is so great 

 during the day that the eye receives little or no impression from an object not 

 brilliantly illuminated. The production of this condition is much favoured by 

 debility and weakness, and it is not of uncommon occurrence in connection with 

 scurvy (see SCURVY). The great thing is to give the eyes rest, and to improve the 

 general health by change of air, the administration of tonics, and so on. Absolute 

 darkness for a few duys,^>r even weeks, till the attack is overcome, is better than 



