October 22, 1891] 



NATURE 



595 



COLOUR-BLINDNESS GENERALLY 

 CONSIDERED. 



/^OLOUR-BLINDNESS has now passed from the 

 ^ category of aihiients denominated interesting, and 

 is recognized as a visual infirmity the importance of 

 which cannot be over-estimated. Before entering upon 

 a discussion of the subject it will be well to lay down a 

 definition of colour-blindness that shall run on all fours 

 with the latest scientific findings in the matter. Colour- 

 blindness is merely the inability of the eye to recognize 

 the quality of the light that falls upon it, i.e. to discriminate 

 between ether waves of varying refrangibility, the im- 

 pingement of which upon the retina conveys to us the 

 sensation of colour. Total colour-blindness is the in- 

 ability to distinguish any colours. To a person so afflicted 

 all bodies are either black, or white, or grey, according to 

 the intensity of the light reflected from them. T lis form 

 of the disease is very rare. Colour-blindness in ordinary 

 is merely a question of degree, no two persons having 

 exactly the same colour perception. A popular, but 

 erroneous, belief respecting human vision is that good 

 eyesight, i.e. accurate perception of form and distance, 

 carries with it a keen perception of colours. This belief 

 is deeply rooted, the impression that colour perception is 

 an integral part of^w</eyesight being of almost universal 

 adoption. The eye, however, that has the most perfect 

 appreciation of form and distance may utterly fail to dis- 

 criminate between two differently coloured objects of the 

 same shape, and placed at equal distances from the 

 observer. In this case a variation in the intensity of the 

 light reflected from the objects under view would enable 

 the colour-blind to discriminate between them, for along 

 with colour ineptitude there generally exists the most 

 delicate sense of discrimination as to the relative inten- 

 sities of two sources of light. 



The majority of people are undoubtedly afflicted with 

 a mild form of colour-blindness. They are physically 

 incompetent to differentiate exactly between the nicer 

 shades of the more composite colours, such as browns, 

 greys, and neutral tints. Yellow would appear to be the 

 colour that gives least trouble to the colour-blind, and 

 blue, if strongly illuminated, is readily recognized. 



Red would appear to be the colour the want of the 

 sense of which may be said to be characteristic of colour- 

 blindness; and as a person blind to red is usually blind to 

 its complementary colour, green, ordinary colour-blindness 

 may thusbedefined as the inability to discriminate between 

 red and green. The norm il eye would appear capable 

 of analyzing white light into three coloured elements, 

 one of which is red ; the colour-blind eye, on the other 

 hand, analyzes white light into two elements, neither of 

 which is red. Why this visual defect should mani- 

 fest itself in inability to distinguish that part of the 

 spectrum which is the result of the slowest of the series 

 of ethereal undulations is by no means clear. Physio- 

 logical knowledge as to the exact relationship between 

 external colour factors and our mental idea of colour is \ 

 yet in its infancy. A consensus of opinion would, how- 

 ever, appear to obtain, that, of the rods and cones to 1 

 which the nerve terminals of the retina are generally ! 

 compared, the latter are responsible for the processes of 

 analysis by which a compound ether wave is decomposed 

 into its constituent elements, each of which pioduces an 

 influence upon a corresponding nerve fibre. That the 

 rods and not the cones are least responsible for our sen- 

 sations of colour would appear to be borne out by the 

 fact that among predatory animals, to whose nocturnal 

 habitat a colour percipient apparatus would be an unne- 

 cessary adjunct, the cones are wanting, while the rods are 

 very highly developed. Which theory may be ultimately 

 accepted as best explaining the varied phenomena of 

 colour-blindness is at present matter of speculation. 

 Authorities on colour-blindness are, however, agreed 

 NO. I 147, VOL. 44] 



that in the majority of cases it is congenital ; that to a 

 great extent it is amenable to the same laws that govern 

 the transmission of other hereditary tendencies ; and 

 while in some very {^^n cases where it is induced by ac- 

 cident, such as concussion of the brain, or is the residual 

 product of some malady or alcoholic excess, it may be 

 palliated, yet colour-blindness is absolutely incurable. 



The knowledge that something like 3 to 4 out of every 

 100 of our adult population are afflicted with colour- 

 blindness is of serious importance, anJ statistics show 

 that this is no over-estimation of the case. The following 

 table shows the percentage obtained from a large number 

 of cases : — 



Number 

 Examined. 



Holmgren 32,165 men 



Dr. Jwyjeff.ies ... 18,556 „ 

 London Committee .. 14,846 ,, 



Number p 



1,019 ••• 3'i68 



764 ... 4-117 



617 ... 4'i56 



Total 



65,567 men 



2,400 



3-66% 



The percentage of female colour-blinds is much less. 

 My own findings show o"i62 ; Dr. Joy Jeffries, however, 

 found a lower proportion than this, as among 14,557 

 females tested only 1 1 were colour-blind. 



This great disparity between the numbers of the colour- 

 blind in the two sexes has been long known, and various 

 causes have been assigned as accountable for it. As far 

 back as 1855, Prof. Wilson, while admitting the superiority 

 of colour perception in the female, could not believe that 

 the number of colour-blind women were so few as com- 

 pared with the number of men similarly afflicted. He 

 took up the view that women were not so willing to be 

 tested as men, so that unless they were members of some 

 public institution it was quite a voluntary matter whether 

 they were tested or not. He argued, too, that women 

 attached greater importance to perception for colour than 

 men do, and would consequently strive to screen their 

 defect from others. Thus the only women who would 

 voluntarily submit to be tested for colour would be those 

 who had no doubt but they were possessed of perfect 

 colour-vision. 



Unfortunately, however, this method of reasoning is 

 based upon an hypothesis altogether fallacious. Colour- 

 blind people do not of themselves realize their condition. 

 They cannot tell that there is any difference between red 

 and green as they see them, and red and green as viewed 

 by the normal eye. 



The fact that females have more practice in handling 

 colours than males is not sufficient to account for this 

 disparity, unless we assume that the present condition of 

 the female colour percipient is the resultant of the gradual 

 development and training transmitted through ages of 

 time. The superior colour percipience of the female 

 must, we believe, be regarded as an inhe'rent quality of 

 the sex, which no amount of individual artificial training 

 and practice can attain to. 



There is just one thing, however, which may discount 

 a httle this feminine superiority. As colour-testing was 

 first applied, too much importance was attached to the 

 correct naming of colours, and as this is a province in 

 which the masculine section of humanity is decidedly 

 inferior, the ratio of male to female colour-blinds may 

 have been increased in consequence. Assuming that the 

 percentage of 3 66 of adult male colour-blinds is correct^ 

 we are confronted with the fact that there are over 4000 

 colour-blind seamen in the British mercantile marine. 

 This number is e.xclusive of pilots, canal or lightermen, 

 and firemen. Now, all of the 4000 are liable to be called 

 upon to officiate as look-outs, i.e. they may be placed in 

 circumstances where it is necessary they should distinguish 

 instantaneously between the colours of the regulation 

 side-lights of an approaching vessel. As far back as 

 1877, the Beard of Trade, acting in accordance with the 



