No. 1072, Vol. 42] 



NATURE 



61 



takes green or gray, or one alone, with or without purple, 

 he is completely green-blind. If he takes red or orange, 

 with or without purple, he is violet-blind. If there be 

 any doubt, the examinee may be subjected to a third test, 

 which is not necessary for the satisfaction of an expert, 

 but which sometimes strengthens the proof in the eyes of 

 a bystander. The pattern for this third test is a skein of 

 bright red, to be used in the same way as the green and 

 the purple. The red- blind fselects for this dark greens 

 and browns, which are much darker than the pattern ; 

 while the green-blind selects greens and browns which 

 are lighter than the pattern. 



The method of examination thus described is, I believe, 

 absolutely trustworthy. It requires no apparatus beyond 

 the bundle of skeins of wool, no arrangements beyond a 

 room with a good window, and a table with a white cloth. 

 In examining large numbers of men, they may be ad- 

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

 their instructions together, and may then make their 

 selections one by one, all not yet examined watching the 

 actions of those who come up in their turn, and thus 

 learning how to proceed. The time required for large 

 numbers averages about a minute a person. I have 

 heard and read of instances of colour-blind people who 

 had passed the wool test satisfactorily, 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 

 pattern colours accurately, and have allowed those whom 

 they examined, 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 maybe fully satisfied by 

 the wool test that the examinee is not capable of dis- 

 tinguishing 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 em- 

 ployer 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 convince 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. Lan- 

 terns for this special purpose were contrived, some years 

 ago, by Holmgren himself, and by the late Prof. Bon- 

 ders of Utrecht, and what are substantially their con- 

 trivances have been brought forward within the last 

 i&w months as novelties, by gentleman in this country 

 who have re-invented them. The principle of all is the 

 same— namely, that light of varying intensity may be dis- 

 played through apertures of varying magnitude, and 

 through coloured glass of varying tints, so as to imitate 

 the appearances of signal lamps at different distances, 

 and under different conditions of illumination, of weather, 

 and of atmosphere. To the colour-blind, the difference 

 between a red light and a green one is not a difiference 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 succession. His liability to error 

 is chiefly conspicuous when he sees one light only, and 

 when the conditions which govern its luminosity depart in 



any degree from those to which he is most accustomed. 

 With the lanterns of which I have spoken, it is always 

 possible to deceive a colour-blind person by altering the 

 luminosity of a light without altering its colour. This 

 may be done by diminishing the light behind the glass, 

 *by increasing the thickness of the red or green glass, or 

 by placing a piece of neutral tint, more or less dark, in 

 front of either. The most incredulous employer may be 

 convinced, by expedients of this kind, that the colour- 

 blind are not to be relied upon for the safe control of 

 ships or of locomotives. With regard to the whole ques- 

 tion, there are many points of great interest, both physical 

 and physiological, which are still more or less uncertain ; 

 but the practical elements have, I think, been well-nigh 

 exhausted, and the means of securing safety are fully in 

 the hands of those who choose to master and to employ 

 them. The lanterns, in their various forms, are useful for 

 the purpose of thoroughly exposing the colour-blind, and 

 for bringing home the character of their incapacity to 

 unskilled spectators ; but they are both cumbrous and 

 superfluous for the detection of the defect, which may be 

 accomplished with far greater ease, and with equal cer- 

 tainty, by the wool test alone. 



I have already mentioned that the examinations which 

 have been conducted in the United States, thanks to the 

 indefatigable labours of Dr. Joy Jeffries, have led to the 

 discovery of an enormous and previously quite unsus- 

 pected amount of colour-ignorance, the condition which 

 is frequently mistaken for colour-blindness by the methods 

 of examination which are in favour with railway com- 

 panies and with the Board of Trade ; and this colour- 

 ignorance has been justly regarded as a blot on the 

 American system of national education. It has there- 

 fore, in some of the States, led to the adoption of 

 systematic colour-teaching in the schools ; and, for 

 this purpose. Dr. Joy Jeffries has introduced a wall- 

 chart and coloured cards. The children are taught, 

 in the first instance, to match the colours in the chart 

 with those of the cards distributed to them ; and, when 

 they are tolerably expert at matching, they are further 

 taught the names of the colours. It must, nevertheless, 

 always be remembered that a knowledge of names does 

 not necessarily imply a knowledge of the things de- 

 signated ; and that colour-vision stands in no definite 

 relation to colour-nomenclature. Even this system of 

 teaching may leave a colour-blind pupil undetected. 



COMPOUND LOCOMOTIVES. 



THE present position of locomotive engineering in this 

 country is of a very interesting nature ; owing to the 

 gradual increase of weight of trains hauled and the higher 

 speeds now in use, it has been necessary to increase the 

 power of the locomotive by leaps and bounds to cope with 

 these demands. This naturally has not been done without 

 great scheming on the part of the designers, for, with the 

 standard gauge of railway of 4 feet 8i inches, the engines 

 are tied down to certain dimensions between the frame 

 plates ; in total length, to a certain extent, by the turn- 

 tables in use ; and in height of boiler for reasons of 

 stability. These questions of design are interesting be- 

 cause they are intimately connected with the economical 

 working of the engines, especially in the consumption of 

 fuel, a question which of late years has taken a prominent 

 position in the economical management of locomotives. 

 For several years the highly economical results obtained 

 at sea with the use of high pressures coupled with the 

 compound or triple expansion engine have caused en- 

 gineers to look in that direction for further improvements, 

 with the result that two different types of compound loco- 

 motives were designed, and are considerably past the ex- 

 perimental stage. These engines are now working suc- 

 cessfully on two of the English railways, and are being 

 adopted on many foreign ones. 



