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.
There is, however, another consideration that this curious subject leads to. It is known that out of every 10,000 rays issuing from the sun, and penetrating space at the calculated rate of 200,000 miles in each second of time, about one-fifth part is altogether lost and absorbed in passing through the atmosphere, and never reaches the outer envelope of the human eye. It is also known that of the rays that proceed from the sun, some produce light, some heat, and some a peculiar kind of chemical action to which the marvels of photography are due. Of these only the light rays are appreciated specially by the eye, although the others are certainly quite as important in preserving life and carrying on the business of the world. Who can tell whether, in addition to the rays of coloured light that together form a beam of white light, four-fifths of which only pass through the atmosphere, there may not have emanated from the sun other rays altogether absorbed and lost? or whether in entering the human eye, or being received on the retina at the back of the eye, or made sensitive by the optic nerve, there may not have been losses and absorptions sufficient to shut out from us, who enjoy what we call perfect vision, some other sources of information. How, in a word, do we who see clearly only three or four colours, and their various combinations, together with their combined white light—how do we know that to beings otherwise organized, the heat, or chemical rays, or others we are not aware of, may not give distinct optical impressions? We may meet one person whose sense of hearing is sufficiently acute to enable him to hear plainly the shrill night-cry of the bat, often totally inaudible, while his friend and daily companion cannot perhaps distinguish the noise
