478 



NATURE 



{Sept. 1 8, 1879I 



might bs brought right by exercise, but a natural and 

 congenital defect of organic structure, altogether in- 

 capable of cure or even amelioration by any means at 

 present known. 



There is every reason to believe the defect is hereditary, 

 and Dr. Jeffries gives some data as to its transmission, 

 showing the curious law (not, however, without exceptions) 

 that it is transmitted through the females, although the 

 females themselves escape it, thus skipping over every 

 other generation. For example, if a man is colour-blind 

 he will have no colour-blind children, neither will any of 

 his sons' sons be colour-blind, but the defect will probably 

 be found among the sons of his daughters. 



It has been asserted by some authorities that a higher 

 percentage of cases exists among people of the Jewish 

 faith, and an ingenious question has been raised whether 

 this may not indicate a descent from an early general 

 type of imperfect vision, so giving a sort of support to 

 the Gladstonian theory of development of the colour- 

 sense. But as the fact is disputed, it is useless to trouble 

 ourselves about any inference from it. 



Another point of interest is the mode of testing and 

 examining the colour-blind, and a large portion of Dr 

 Jeffries', as of all other late works, is devoted to this 

 matter. There are two distinct objects in view in such 

 examinations : first, simply to discover whether the 

 e.xaminee is or is not colour-blind ; secondly to find out 

 the precise nature of his sensations of colour. 



The first is of the more practical importance, and it is 

 aimed, of course, chiefly at the testing of railway servants 

 and others who have to do with colours. I hold a strong 

 opinion that too much alarm has been created about the 

 danger of mistaking railway signals. Normal-eyed people 

 (who have always the greatest difficulty in comprehending 

 what the colour-blind really see) generally believe that 

 because under certain circumstances a red and a green 

 object give similar impressions to a Daltonian, therefore 

 he must, under all circumstances, confound redness and 

 greenness. Nothing can be further from the truth, and I 

 may give my own experience with railway signals as an 

 example. I have had a great deal to do with railways, 

 and although there can be no doubt about my colour- 

 blindness, I do not recollect that I ever experienced any 

 difficulty in distinguishing a red lamp from a green one. 

 They are always strongly contrasted to my eye, and 

 often, when I have passed at night through Cannon 

 Street Station, I have amused myself by watching the 

 changes in the imposing array of signals exhibited 

 there. I could give a sufficient explanation of this, but 

 it would be out of place here. I believe, so far as my 

 knowledge at present goes, that nearly all colour-blind 

 patients see the distinction as well as I do ; and this view 

 is corroborated by the great fact that, although we may 

 assume that down to a late period about one out of every 

 twenty-five engine-drivers has been colour-blind, never 

 since railways have been in use has a single accident 

 occurred which has been positively attributable to the 

 mistaking of a red for a green night signal. If the notions 

 of the alarmists were well founded, we should have had' 

 collisions every day. At the same time I do not deny the 

 possibility of danger, under certain circumstances, and I 

 would by no means discourage the precautions proposed 

 in the selection of men. 



It is not so easy a matter to identify positively a case 

 of colour-blindness as might at first be supposed. Mere 

 inaccuracy in naming colours is not sufficient, for it often 

 happens, on the one hand, that normal-eyed people 

 name colours incorrectly, and, on the other hand, that 

 colour-blind people will name many colours correctly. 

 There have been several kinds of tests adopted by dif- 

 ferent examiners, but the most important are those by 

 Holmgren, Schilling, and Daae. The first is the one 

 recommended by Dr. Jeffries, and it is described by him 

 at considerable length. The test is made with a large 

 number of samples of worsted (or what we call Berlin- 

 wool) of a great variety of colours and shades. A 

 certain sample, of a pale but decided tint of green, is 

 shown to the patient, who is desired to select from the 

 heap all the specimens which, to his eye, match it in 

 colour. If he is normal-eyed he will select only green 

 samples, but if he is colour-blind he will add others of 

 other colours, such as pink, light-brown, gray, &c., which, 

 though so different to the normal-eyed, match the green tint 

 to his defective vision. But the wools have to be carefully 

 selected, and are expensive. Dr. Schilling's test consists 

 of a set of lithographed coloured diagrams ; he takes two 

 colours which, though normally strongly contrasted, he 

 knows appear alike to the colour-blind, say scarlet ami 

 yellow-brown, and he draws letters or patterns in one of 

 these, on a ground of the other, i.e., a scarlet pattern on 

 a brown ground, or vice versd, the design being so in- 

 geniously arranged as to avoid betraying any lines of 

 division. A single glance at these diagrams by a colour- 

 blind person suffices to test his vision. If he is normal- 

 eyed the patterns are visible to him ; if he is colour-blind 

 they are invisible, the whole diagram conveying to his 

 mind the idea of one uniform colour. 



Dr. Daae's test is a little page of samples of coloured 

 worsted, arranged in rows. Some of the rows are devoted 

 each to one colour, arranged in different tints and shades, 

 while other rows contain different colours in the same 

 row. If the patient is unable to distinguish between 

 these two classes he is colour-blind. 



All these tests are very simple, and appear to be efficient 

 so far as the mere detection of ths fact is concerned, 

 which is all that is wanted for economic purposes. 



But some of the investigators go farther ; they prescribe 

 other tests with a view to find out what is the nature of 

 the patient's vision — to get an idea of what he actually 

 sees. The arrangements proposed for this purpose are 

 much more elaborate. Holmgren, for example, exhibits 

 other samples of wool, and endeavours to infer the nature 

 of the vision by observing what colours are considered to 

 match it. Schilling has different sets of diagrams for 

 what he considers different classes of the complaint, 

 while many other contrivances, some of them extremely 

 complicated, have been designed to test the vision by 

 shadows, by coloured glasses, by reflected images ; by 

 polarisation, by contrast, by comparisons, and by the 

 spectroscope. These are all more or less unsatisfactory, 

 and the information hitherto obtained by them is worth 

 very little. The answers of the patients are seldom 

 trustworthy, and they are almost always interpreted 

 by the examiner to suit some preconceived theory of his 

 own. One of the most earnest and industrious investi- 

 gators, Dr. Cohn, gives a long list of answers he has 



