Page:Popular Science Monthly Volume 50.djvu/776

754 to the eye. Objects beyond our reach, as the stars or the clouds, are not truthfully pictured. Accuracy of perception grows less as distance increases. The unfamiliar lends itself readily to illusions; the familiar is recognized chiefly by breaks in continuity. The real forces of Nature are hidden by their grandeur, by their immortality. Men see the form of the surface, but not the mighty tides that move beneath it. Again, the senses are less acute than the mechanism of sense organs would make possible. This is shown through occasional cases of hyperæsthesia or ultra sensitiveness. This occurs in abnormal individuals or in unusual conditions. It occurs normally in creatures whose lives in some sense depend on it. Thus some of the most remarkable exhibitions of "mind reading" may be paralleled by retriever dogs whose reason for existence is found in the hyperæsthesia of the sense of smell. Hyperæsthesia of any of the senses would be to most animals a source of confusion and danger rather than of safety.

Man's high development of the brain in large degree takes the place of acuteness of special senses. It is part of the function of the will to keep down the senses; and in his perception of external relations he is aided by the devices of science, which may be taken up or laid down at will. By means of instruments of precision any of the senses may be aided to an enormous degree, and at the same time the personal equation or individual source of error is largely eliminated. The use of instruments of precision is the special characteristic of the advance of science. No instrument of precision can give us the ultimate essence of any part of the universe. No scientific experiment can do away with the measure of human experience as the basis of intelligibility. At the same time we can throw large illumination into "the dimly lighted room" in which, according to Balfour, the phenomena of consciousness take place. By the simple process of photography, for example, we may reproduce the objects of our environment. That such pictures do express phases of reality admits of no doubt, for in the photographic camera all personal equation is eliminated. As to form of outline and reflection of light, the "sun paints true," and the paintings thus made by means of the action of nonliving matter produce on our senses impressions coinciding with those of the outside world itself.

How do we know this is true? Because belief in it adds to the safety of life. We can trust our lives to it. If it were an illusion it would kill, because action based on illusion leads to death.

One can trust his life, for example, to the message sent on a telegraph wire. All who travel by rail do this daily. One can trust his life to the reading of a thermometer. The chemist's