284 Beport of the Committee on Colour-Vision, 



green; that is to say, a red-green blind person will regard a 

 certain hue of green as identical in colour with some hue of red r 

 another of green as identical with white, and some will also 

 fail to see red at all of another particular hue. When it is 

 considered that on our railways white, green, and red lights are 

 used as safety and danger signals at night, and that the same 

 colours are not unfrequently used for a similar purpose by day, 

 it is very obvious that to place persons who are red-green blind 

 in positions where the colours ought to be correctly recognised 

 may be the cause of disasters. The same objection to the employ- 

 ment of persons with defective colour-vision applies also to 

 navigation, for at night the presence of a green or red light on 

 the port or starboard side indicates the course that a vessel is 

 taking, and if either those in charge, or on the look-out, are 

 colour-blind, serious risks of collisions are run. 

 Description of It is proposed to enter somewhat minutely into the 

 the spectrum, characteristics of red-green blindness, showing how it may be 

 divided into two species. For this purpose it is necessary 

 to appeal to the spectrum. When a thin slice of white 

 light falls on one or more prisms, or on what is known as a 

 diffraction grating, it is decomposed into a parti- coloured band 

 which we call the spectrum, the principal colours, as given by 

 Newton, being red, orange, yellow, green, blue, indigo and violet. 

 If the light be that from the sun innumerable black lines will 

 be seen interrupting this series of colours, some more marked 

 than others. It is found that these lines always occupy the 

 same position as regards the colour in which they are situated, 

 and hence the more pronounced ones will act to the spectrum as 

 milestones do to a road. Different coloured rays have different 

 lengths of undulations in the all-pervading medium which is 

 called ether, and the wave lengths of the coloured rays which, 

 if present, would occupy the place of the principal black lines 

 have, notwithstanding their minuteness, been determined with 

 extreme accuracy, and this enables the position of any particular 

 hue of spectrum colour to be numerically fixed by a reference to 

 the wave lengths of these lines. We have said that the prin- 

 cipal spectrum colours are those stated above, but it must be 

 understood that they are only fully recognized by persons 

 possessing normal vision; for the spectrum would be described 

 by a colour-blind person in very different terms. For instance, 

 some red-green blind would say that the red, orange, 

 and yellow were all yellow; red would be described as 

 dark yellow, orange as less dark, and yellow as bright yellow, 

 whilst the green part of the spectrum bordering on the yellow 

 would be described as yellow diluted with white. In the pure 

 green would be pointed out a white or grey band, and the blue- 

 green would be described as blue diluted with white ; whilst 

 the blue would be called light blue, and the violet dark blue 

 {see No. 2, Plate I). Others, again, whilst similarly describing 

 the blue and violet part of the spectrum would substitute green 



