Report of the Committee on Colour- Vision. 293 



relative brightness, and by their dilution with neutral colour. 

 Thus, a bluish-green signal might be distinctly known by 

 its blue hue, whilst if yellowish-green, it might be recognized 

 by the neutral colour being slightly tinged with the only other 

 spectrum colour which they see. Again, a green whose hue, 

 whether pure or diluted with white, accurately coincides with 

 that part of the spectrum where the neutral band is situated, 

 might probably be mistaken for white, though, even from that, 

 it mig'ht be distinguished by its lower luminosit}^. The practical 

 tests the Committee have carried out confirm this view ; men 

 who are absolutely colour-blind having* passed such a test 

 without being detected. It might be supposed that if the 

 colours of signals could be rightly recognized in the testing- 

 room they would be equally well recognized elsewhere. It 

 must, however, be recollected that the atmospheric conditions of 

 the testing-room are often very different from those which are 

 found outside. As a rule any judgment of the colour of a signal 

 which depended upon its brightness would be fallacious. A 

 dirty glass, or a misty atmosphere, would introduce a liability to 

 error. The red signal of danger might then be mistaken for 

 the green or white signal of safety, and vice versa. It must 

 also be remembered that a signal light, as a rule, has no 

 white light adjacent to it with which to compare it, and thus a 

 decision as to whether a light is neutral, or slightly coloured, 

 has to be arrived at under great disadvantages. We shall 

 presently call attention to the conditions which regulate the 

 choice of the colours to be used as signals ; here it is sufficient to 

 say that, even if a green were used, whose dominant spectrum 

 colour lay on the blue side of the neutral bands, mistakes might 

 still occur, more particularly in certain conditions of foggy 

 weather, when white light in its passage is deprived of the blue 

 rays in greater proportion than the green, and the green in 

 greater proportion than the red {see Note d, page 305). 



We have so far confined our attention to colour-blind vision Description of 

 of the dichromatic type. Incomplete colour-blindness is less incomplete 

 likely to lead to accident than that which is complete; but any ?° lo ^ r " 

 colour-blindness, in which there is approximately a neutral or blmdness - 

 grey point in the spectrum, should be regarded with great sus- 

 picion. On the other hand, there are many people who have a 

 slightly shortened spectrum, who are yet able to distinguish all 

 colours, and see no neutral point. These cannot be considered 

 to be practically colour-blind. There are again others to whom 

 the spectrum is considerably shortened, but not to the extent 

 that it is in complete red-blindness, and they have what is ap- 

 parently a neutral point in the spectrum, lying very close to 

 that which is found in the complete colour-blind cases. The 

 presence of this neutral colour points to such a degree of im- 

 perfection in colour sense that it must be classed as dangerously 

 defective. A certain and prompt recognition of a green signal 

 colour by these last would undoubtedly be difficult under some 



