Report of the Committee on Colour-Vision. 305 



day-light. As this is the case with the normal-eyed, it may 

 be assumed that the same mental standard of whiteness would 

 be adopted by the colour-blind. 



Note (d). 



In discussing- the most suitable colour of signals, the question Effect of fog 

 of the possible alteration of hue by the interposition of fog on the colour 

 between them and ths observer must be taken into account. of signals. 

 There are white fogs and yellow fogs, the difference between the 

 two being chiefly in the size of the particles of water, dust, or 

 soot which are to be found in them. In a white fog away from 

 large towns the particles are chiefly water, but whilst the great 

 majority must be large compared with the length of a wave of 

 light, yet some will be present which are very much smaller. In a 

 yellow fog the fine particles are much more largely present, and 

 the yellowness is largely due to this fact, for when particles, 

 whose sizes are comparable to a wave-length of light, are present 

 between the source of light and the observer, the law of scattering 

 requires that the blue part of the spectrum of the light reaching 

 the latter should be much more enfeebled than the green, the green 

 than the yellow, and the yellow than the red. A blue-green 

 signal glass will therefore appear rather less blue in a white fog', 

 and even yellowish-green in a yellow fog, and it may happen 

 that the loss of what are blue and green to the normal eye 

 will shift the colour of the signal to the red side of the neutral 

 point in the spectrum of each type of a colour-blind person, and 

 then both red and green signals will appear of the same tint 

 to him, though the latter will appear more diluted with the 

 neutral colour. It follows therefore that in a fog the liability 

 of the colour-blind to mis-read signals is very much greater 

 than in ordinary clear weather. 



