All colour-blind persons do not seem to make exactly the same mistakes, or see colours in the same way; and there are, no doubt, many minor defects in appreciating, remembering, or comparing colours which are sufficiently common, and which may be superadded to the true defect—that of the optic nerve being insensible to the stimulus of pure red light. It has been asserted by Dr. Wilson, the author of an elaborate work on the subject, that as large a proportion as one person in every eighteen is colour-blind in some marked degree, and that one in every fifty-five confounds red with green. Certainly the number is large, for every inquiry brings out several cases; but, as Sir John Herschel remarks, were the average anything like this, it seems inconceivable that the existence of the defect should not be one of vulgar notoriety, or that it should strike almost all uneducated persons, when told of it, as something approaching to absurdity. He also remarks, that if one soldier out of every fifty-five was unable to distinguish a scarlet coat from green grass, the result would involve grave inconveniences that must have attracted notice. Perhaps the fact that a difference of tint is recognized, although the eye of the colour-blind person does not appreciate any difference of colour, when red, green, and other colours are compared together, and that every one is educated to call certain things by certain names, whether he understands the true meaning of the name or not, may help to explain both the slowness of the defective sight to discover its own peculiarity, and the unwillingness of the person of ordinary vision to admit that his neighbour really does not see as red what he agrees to call red.