132 Mr. R. Briidenell Carter [May 9, 



trustworthy. It requires no apparatus beyond tlie bundle of skeins of 

 wool, no arrangements beyond a room witb a good window and a 

 table with a white cloth. In examining large numbers of men, they 

 may be admitted into the room fifty or so at a time, may all receive 

 their instructions together, and may then make their selectious one 

 by one, all not yet examined watcLing the actions of those who come 

 up in their turn, and thus learning how to proceed. The time re- 

 quired for large numbers averages about a miuute a person. I have 

 heard and read of instances of colour-blind people who had passed 

 the wool test satisfactory, and had afterwards been detected by other 

 methods ; but I confess that I do not believe in them. I do not 

 believe that in such cases the wool test was applied properly, or in 

 accordance with Holmgren's very precise instructions ; and I know 

 that it is often applied in a way which can lead to nothing but 

 erroneous results. Railway foremen, for example, receive out of 

 store a small collection of coloured wools selected on no principle, 

 and they use it by pulling out a single thread, and by asking the 

 examinee, " What colour do you call that ? " Men of greater scientific 

 pretensions than railway foremen have not always selected their 

 imttern colours accurately, and have allowed those whom they ex- 

 amined, and passed, to make narrow comparisons between the skeins 

 in all sorts of lights, in a way which should of itself have afforded 

 sufficient evidence of defect. 



Although, however, the expert may be fully satisfied by the wool 

 test that the examinee is not capable of distinguishing with certainty 

 between red and green flags or lights in all the circumstances in 

 which they can be displayed, it may still remain for him to satisfy 

 the employer who is not an expert, the railway manager, or the ship- 

 owner, and to convince him that the colour-blind person is unfit for 

 certain kinds of employment. It may be equally necessary to con- 

 vince other workmen that the examinee has been fairly and rightly 

 dealt with. Both these objects may be easily attained, by the use 

 of slight modifications of the lights which are employed. Lanterns 

 for this special purpose were contrived, some years ago, by Holmgren 

 himself, and by the late Prof. Bonders, of Utrecht, and what are 

 substantially their contrivances have been brought forward within the 

 last few months as novelties, by gentlemen in this country who have 

 re-invented them. The principle of all is the same — namely, that 

 light of varying intensity may be disi)layed through apertures of 

 varying magnitude, and through coloured glasses of varying tint, so 

 as to imitate the appearances of signal lam])S at different distances, 

 and under different conditions of illuminatirn, of weather, and of 

 atmosphere. To the colour-blind, the difierence between a red light 

 and a green one is not a difference of colour, but of luminosity ; the 

 colour to which he is blind appearing the less luminous of the two. 

 He may therefore be correct in his guess as to which of the two is 

 exhibited on any given occasion, and he is by no means certain to 

 mistake one for the other when they are exhibited in immediate 



