There was a time when one could catalogue all the physical forces which altered our environment quite neatly. These were forces of nature such as heat, cold, wind, and water which, for the most part, could be readily seen or felt and their benefits and/or dangers appreciated. These mechanical and thermal forces have been so long a part of everyday existence that during the course of organic evolution special neurosensory devices have developed in the skin to warn of excesses of heat or of mechanical injury. Subtle physical forces such as ultraviolet radiation and natural radioactivity were never of such acute importance as to condition the body to evolve special sensory nerve endings to see or feel them. The organism did develop special sensory devices for detecting electromagnetic waves but only in the narrow visible region and audible wavelengths since these had obvious practical importance.
In recent years the application