This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
Colour Blindness is of three sorts. Some persons have been found unable to appreciate any difference of colour at all, to whom the world was like an engraving. A number of persons have an inability to distinguish allied tints one from another, and confuse blue with green, or confound pinks, crimsons, and scarlets together. In a third set of cases, colours totally unlike are confounded; and the most remarkable colourblindness of this description consists in inability to distinguish red from green or blue. Curiously enough, this defect may exist without the object of it having any suspicion that his vision is defective. Thus, Dalton, the celebrated chemist, from whom colour-blindness occasionally gets the name of Daltonism, was twenty-five years of age before he was distinctly convinced of his peculiarity of vision. Yet so great was this peculiarity that in describing it he wrote: " Crimson appears a muddy blue by day, and crimson woollen yarn is much the same as dark blue;" and further recorded that the one side of a laurel leaf seemed to him a good match to a stick of sealing-wax, and the other side to a red wafer. After Dalton had attracted attention to the matter, it was found that his case was so far from being solitary that colour-blindness in different degrees was not unfrequently to be met with. This being the case, it is evident that, on railways and at sea, danger signals dependent on the number or position of lights are preferable to those dependent on colour, and that red lights are specially objectionable.