1890.] on Colour-Vision and Colour-Blindness. 133 



succession. His liability to error is chiefly conspicuous when he 

 sees one light only, and when the conditions which govern its lumi- 

 nosity 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 [shoion^. 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 question, 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 ex- 

 posing 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 certainty, 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 unsuspected amount of colour-ignorance, a condition 

 which is frequently mistaken for colour-blindness by the methods 

 of examination which are in favour with railway companies 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 therefore, 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 this wall-chart and coloured cards [s/iOi(7?i]. 

 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 designated ; and that colour- vision stands in no definite 

 relation to colour-nomenclature. Even this system of teaching 

 may leave a colour-blind pupil undetected. 



[E. B. C] 



