294 Report of the Committee on Colour-Vision 



Colour-blind- 

 ness induced 

 by disease. 



Colour-blind 

 persons 

 should be 

 rejected for 

 certain occu- 

 pations. 



Most suitable 

 colours for 

 signals, and 

 causes which 

 modify their 

 sslection. 



conditions of atmosphere, or if the mind were disturbed by some 

 imminent danger. 



In colour-blindness, induced by disease or injury, although the 

 loss of colour sense is usually confined to a small area of the 

 retina, yet, as it is the central area, and therefore the part on 

 which the image of small objects naturally falls, the danger of 

 mistaking a colour is as great, and even more so than in congenital 

 colour-blindness ; for loss of colour-sense is in this case as already 

 has been stated accompanied by loss of form-sense. 



On the general grounds that have been explained, the Com- 

 mittee are of opinion that it would, under any circumstances, 

 be dangerous to trust the reading of signals to anyone who is 

 totally or even partially colour-blind to the extent indicated 

 above, and this opinion is fortified by practical tests which they 

 have carried out. They consider that such a person under no 

 circumstances should be allowed to take a post for which this 

 defect renders him physically unfit, and with this object in view 

 the tests employed in the examination should be of a nature to 

 at once detect, not only pronounced colour-blindness but defective 

 colour-vision of the above character. 



On some railways white lights instead of green have been used 

 as safety signals, but the former are liable to be confounded with 

 other white lights which are not signals, more particularly in the 

 neighbourhood of towns. At sea the evidence shows that the 

 use of a second coloured light in addition to a red is a necessity, 

 and that a white light could not be substituted for it. 



It has been suggested, on theoretical grounds, that all danger 

 of misreading signals would be avoided by using for one a 

 red and for the other a pure blue, as each of these colours is 

 recognized by the red-green blind. Certain difficulties, however, 

 present themselves in practice which preclude the employment 

 of the blue, more especially for night signals. The desiderata 

 for signals are, that they should be as bright as possible, and 

 that their colour should be distinct when viewed at a distance. 

 A red glass transmits about 10 per cent, of the luminosity of 

 the lamp-light behind it ; it is also a saturated colour, and 

 appears unaltered in hue from whatever distance it may be 

 viewed. A blue glass, as ordinarily met with, will appear purple, 

 or even whitish, by lamp-light, as it transmits, besides blue, 

 a large proportion of red rays, and, if it be pale, it will also 

 transmit a variable quantity of all the colours of the spectrum : 

 moreover, the luminosity of the light transmitted is, at the 

 best, only some 4 per cent, of the naked light. If two glasses, 

 one of blue -green and another of cobalt blue, be placed together, 

 in front of the light, the red rays will be cut off, and the light will 

 be a fairly pure blue, but the luminosity will be reduced to about 

 2 per cent. When the effect of foggy weather on the carrying- 

 power of different lights is considered {see Note a, page 303), 

 it will be understood how this small luminosity will be again 

 diminished, and that it will become practically nil. In making 



