6o 



NA rURE 



[May 15, 1890 



thousand dififerent colours, each with its more or less 

 appropriate name. There is here a larger colour-vocabu- 

 lary than the entire vocabulary, for the expression of all 

 his knowledge and of all his ideas, which is possessed by 

 an average engine-driver or fireman; and, just as most of us 

 would be ignorant of the names of the immense majority of 

 the colours displayed on that card, so hundreds of men and 

 boys among the labouring classes, especially in large towns, 

 where the opportunities ofeducation by the colours of flowers 

 and insects are very limited, are ignorant of the names of 

 colours which persons of ordinary cultivation mention con- 

 stantly in their daily talk, and expect their children to pick 

 up and to understand unconsciously. It is among people 

 thus ignorant that the officials of the Board of Trade, and 

 of railways, have been most successful in finding their 

 supposed colour-blind persons ; and these persons, who 

 would never have been pronounced colour-blind by an 

 expert, have been able, as soon as they have paid a little 

 attention to the observation and naming of colour, to 

 pass an official examination triumphantly. The sense of 

 colour presents many analogies to that of hearing. Some 

 people can hear a higher or a lower note than others, the 

 difference depending upon structure, and being incapable 

 of alteration. No one who cannot hear a note of a certain 

 pitch can ever be trained to do so ; but, within the original 

 auditory limits of each individual, the sense of hearing 

 may be greatly improved by cultivation. In like manner, 

 a person who is blind to red or green must remain so ; 

 but one whose colour-sense is merely undeveloped by 

 want of cultivation may have its acuteness for fine 

 differences very considerably increased. 



In order to test colour- vision for railway and marine 

 purposes, the first suggestion which would occur to many 

 people would be to employ as objects the flags and signal 

 lanterns which are used in actual working. I have heard 

 apparently sensible people use, with reference to such a 

 procedure, the phrase upon which Faraday was wont to 

 pour ridicule, and to say that the fitness of the suggested 

 method " stands to reason." To be effectual, such a test 

 must be applied in different states of atmosphere, with 

 coloured glasses of various tints, with various degrees of 

 illumination, and with the objects at various distances ; 

 so that much time would be required in order to exhaust 

 all the conditions under which railway signals may pre- 

 sent themselves. This being done, the examinee must be 

 either right or wrong each time. He has always an even 

 chance of being right ; and it would be an insoluble 

 problem to discover how many correct answers were due 

 to accident, or how many incorrect ones might be 

 attributed to nervousness or to confusion of names. 



We must remember that what is required is to detect 

 a colour-blind person against his will ; and to ascertain, 

 not whether he describes a given signal rightly or wrongly 

 on a particular occasion, but whether he can safely be 

 trusted to distinguish correctly between signals on all 

 occasions. We want, in short, to ascertain the state of 

 his colour-vision generally ; and hence to infer his fitness 

 or unfitness to discharge the duties of a particular occu- 

 pation. 



For the accomplishment of this object, we do not in 

 the least want to know what the examinee calls colours, 

 but only how he sees them, what colours appear to him 

 to be alike and what appear to be unlike ; and the only 

 way of attaining this knowledge with certainty is to cause 

 him to make matchesbetween coloured objects, to put those 

 together which appear to him to be essentially the same, 

 and to separate those which appear to him to be essentially 

 different. This principle of testing was first laid down 

 by Seebeck, who required from examinees a complete 

 arrangement of a large number of coloured objects ; but 

 it has been greatly simplified and improved by Prof. 

 Holmgren, who pointed out that such a complete ar- 

 rangement was superfluous, and that the only thing 

 required was to cause the examinee to make matches to 



certain test colours, and, for this purpose, to select from 

 a heap which, contained not only such matches but also 

 the colours which the colour-blind were liable to confuse 

 with them. 



After many trials, Holmgren finally selected skeins of 

 Berlin wool as the material best suited for this purpose ; 

 and his set of wools comprises about 150 skeins. The 

 advantages of his method over every other are that the 

 wool is very cheap, very portable, and always to be 

 obtained in every conceivable colour and shade. The 

 skeins are not lustrous, so that light reflected from the 

 surfaces does not interfere with the accuracy of the 

 observation ; and they are very easily picked up and 

 manipulated, much more easily than coloured paper or 

 coloured glass. The person to be tested is placed before 

 a table in good daylight, the table is covered by a white 

 cloth, and the skeins are thrown upon it in a loosely 

 arranged heap. The examiner then selects a skein of 

 pale green, much diluted with white, and throws it down 

 by itself to the left of the heap. The examinee is directed 

 to look at this pattern skein and at the heap, and to pick 

 out from the latter, and to place beside the pattern, as 

 many skeins as he can find which are of the same colour. 

 He is not to be particular about lighter or darker shades, 

 and is not to compare narrowly, or to rummage much 

 amongst the heap, but to select by his eyes, and to use 

 his hands chiefly to change the position of the selected 

 material. 



In such circumstances, a person with normal colour- 

 sight will select the greens rapidly and without hesitation, 

 will select nothing else, and will select with a certain 

 readiness and confidence easily recognized by an ex- 

 perienced examiner, and which may even be carried to 

 the extent of neglecting the minute accuracy which a 

 person who distrusts his own colour-sight will frequently 

 endeavour to display. Some normal-sighted people will 

 complete their selection by taking greens which incline to 

 yellow, and greens which incline to blue, while others will 

 reject both ; but this is a difference depending sometimes 

 upon imperfect colour education, sometimes upon the in- 

 terpretation placed upon the directions of the examiner, 

 but the person who so selects sees the green element 

 in the yellow-greens and in the blue-greens, and is not 

 colour-blind. The completely colour-blind, whether to 

 red or to green, will proceed with almost as much speed 

 and confidence as the colour-sighted ; and will rapidly 

 pick out a number of drabs, fawns, stone-colours, pinks, 

 or yellows. Between the foregoing classes, we meet with 

 a few people who declare the imperfection of their colour- 

 sense by the extreme care with which they select, by their 

 slowness, by their hesitation, and by their desire to com- 

 pare this or that skein with the pattern more narrowly 

 than the conditions of the trial permit. They may or 

 may not ultimately add one or more of the confusion 

 colours to the green, but they have a manifest tendency 

 to do so, and a general uncertainty in their choice. One 

 of the great advantages of Holmgren's method over overy 

 other is the way in which the examiner is able to judge, 

 not only by the final cl.oice of matches, but also by the 

 manner in which the choice is made, by the action of the 

 hands, and by the gestures and general deportment of the 

 examinee. 



When confusion colours have been selected, or when 

 an unnatural slowness and hesitation have been shown in 

 selecting, the examinee must be regarded as either com- 

 pletely or incompletely colour-blind. In order to deter- 

 mine which, and also to which colour he is defective, he is 

 subjected to the second test. For this, the wool is mixed 

 again, and the pattern this time is a skein of light purple — 

 that is, of a mixture of red and violet, much diluted with 

 white. To match this, the colour-blind always selects 

 deeper colours. If he puts only deeper purples, he is in- 

 completely colour-blind. If he takes blue or violet, either 

 with or without purple, he is completely red-blind. If he 



