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 of the grasshopper, or the croaking of frogs, and yet neither of these differs sufficiently from the generality of mankind to attract attention, and both may pass through life without finding out their differences in organization, or knowing that the sense of hearing of either is peculiar. So undoubtedly it is with light. There may be some endowed with visual powers extraordinarily acute, seeing clearly what is generally altogether invisible; and this may have reference to light generally, or to any of the various parts of which a complete sunbeam is composed. Such persons may habitually see what few others ever see, and yet be altogether unaware of their powers, as the rest of the world would be of their own deficiency.