Research in Medical Sciences

Abstract

THE experiments of Faraday on electricity and of Helmholtz on heat and on light were incorporated during the latter half of the nineteenth century into a comprehensive electromagnetic theory inter relating light and electric and magnetic fields. Clerk Maxwell (1831-79) proposed the theory that light waves consist of oscillating electric fields at right angles to which are magnetic fields. The velocity of light, three hundred million meters (or 186,000 miles) per second, is the same for impulses of short and long wave-lengths and is one of the most impor tant of physical constants, comparable in its universality to the force of gravity. Indeed Einstein, in 1905, deduced, in terms of the velocity of light, the relation between mass and energy which made possible the prediction of atomic fission. Since the velocity is the same for all electromagnetic waves, includ ing light, differences in property are associated with the length of the wave or with its frequency. The propagation of electromagnetic waves, first studied by Hertz (1857-94), led to wireless telegraphy. Improve ments in technique have made it possible to transmit both very long and very short wave-lengths, ranging from centimeters to thousands of meters. The bands of the modern radio generally extend from 150 to 600 meters. Waves of these lengths may be absorbed by biological sys tems and give rise to considerable heating effects. Radio waves are very long in comparison with those of visible light, which range from about 0.0000007 meters, characteristic of red light, to about 0.0000004 meters for violet light. For convenience, different units are generally used in describing waves of short length, namely Angstrom units, one Angstrom unit being equal to one billionth of a meter. In these terms, red light has a wave length of 7,000 and violet light of 4,000 Angstrom units. The visibility of cells through the microscope depends upon the fact that their diameters are of the same order as the wave-length of light. Objects smaller than cells cannot be observed with ordinary light even through the most powerful microscope. In 1895, Wilhelm R?ntgen (1845-1923), studying the discharge of electricity through gases, noted that photographic plates, placed near the tubes, fogged when the gas in the tubes was largely exhausted. The action was not due to light, but to very short electromagnetic waves which passed through the tubes. The chance nature of the observation of this previously unknown type of radiation, X ray, is reminiscent of Galvani's earlier observation of the nature of electricity and of Bec querel^ later observation of radioactivity. In each instance, discoveries in new fields of the greatest importance were made because, to quote the