It is well known that but a portion of the phenomena of nature are cognizable by us—how small a portion it is not known. A person born blind has no conception of color; one born deaf, none of sound. The disease of color-blindness affords a good illustration of the limitation of knowledge in consequence of the limitation of sensibility in certain directions. One affected with this disease, while able to see outlines with perfect distinctness, as well as some of the colors, is insensible to others; and this insensibility does not affect alone the ability to discriminate between colors nearly alike, but also those that are apparently widely divergent. It is related of Dalton, from whom color-blindness takes its name of daltonism, that some wag exchanged the robe of sober drab which this demure Quaker was in the habit of wearing, for one of scarlet, and that, unconscious of the difference, he went forth, much to the amusement of the bystanders. Undoubtedly many accidents are due to the inability to discriminate between different colored signals. This subject is just beginning to receive the attention its importance demands, and it is found much more prevalent than was even suspected. Like short-sightedness and, probably, all maladies due to the absence or the imperfection of any of the senses, the subject of it is unconscious of its existence until something particular calls it to his attention. To a greater or less extent we may all be said to be color-blind. Only a part of the rays that are known to proceed from the sun are visible to any of us. Vision is produced only by waves of a certain definite length. The rays of longer wave-length than the red are known to us by their heat producing effects only; those of shorter wave-length than the violet, by their chemical effects. It is not improbable that, at some future time, a race may exist capable of seeing rays invisible to us—just as we perceive many invisible to some of the lower animals. In the development of sight there must, at first, have been simply a power to distinguish light from darkness; then, the ability to distinguish outlines; and, finally, the sensibility to color. Until this sensibility was developed, objects would present merely an alternation of light and shade, similar to the view we get in a stereoscopic picture.
Sounds inaudible to us are heard by insects, and odors that we can not distinguish are smelled by animals. So we might go through the whole category of sensations and resulting ideas, and show their variation under different conditions; and yet, whatever may be the theory of knowledge we espouse, it is from these sensations and ideas that we form our conception of nature. How different our mental conceptions would be if our senses were indefinitely magnified, we can but guess. A drop of water, it has been estimated by Sir William Thomson, if magnified to the size of the earth, would be seen to consist of molecules of the size of cricket-balls; and these molecules are themselves of great complexity. The internal motions of a drop of blood are more complex than the motions of the solar system. Place