ULTRAVIOLET LIGHT — MEIER 351 



in sunlicrht -would kill the bacteria present in decaying organic 

 matter. They erroneously believed that free oxygen was necessary 

 to accomplish the bactericidal effect, just as Duclaux (1885), Roux 

 (1887), Ward (1893), and Wesbrook (1895) concluded when they 

 exposed cultures of disease bacteria to sunlight. 



One of the early investigators who attempted an exact quantita- 

 tive analysis of the biological properties of light was Hertel (1905). 

 His monochromator with its quartz prism and lenses was similar to 

 those now used in ultraviolet microscopy. B\ means of a thermopile 

 he determined the relative intensities of four lines of the ultraviolet 

 part of the spectrum, using the dispersed energy emitted from 

 various metals under the influence of the electric arc, and he varied 

 the intensity by regulating the amperage of the metallic arc. He 

 found that the shorter the wave length, the greater the biological 

 effect of the light on microorganisms. Hertel found that in the 

 region of wave lengths around 2800 A, or about the shortest wave 

 lengths of the sun's spectrum that can penetrate the ozone of the 

 atmosphere, and at still shorter wave lengths, there was a very 

 destructive action on bacteria and paramoecia (single-celled .micro- 

 scopic animals). 



Since the pioneer work of Hertel, the lethal action of the invisible 

 ultraviolet rays on those micro-organisms so small that their outlines 

 are made visible to the naked eye only with the aid of a microscope 

 has grown to be a subject of increasing interest to research workers 

 all over the world. The names of these scientists are too numerous 

 for individual mention. They have recognized in these short rays 

 an invisible battalion of death and destruction to microscopic plant 

 and animal life. With scientific foresight and patience, they are 

 utilizing the lethal power concentrated in such rays to destroy the 

 pernicious disease bearers, an invisible but deadly host. 



Ccrnovodcanu and Henri (1910), working with bacteria that cause 

 typhus, dysentery, cholera, and tetanus, found that the bactericidal 

 rays were those in the ultraviolet below 2900 A, which are absorbed 

 by the protoplasm of the cell. These ultraviolet rays produce on 

 the protoplasm certain chemical and physical transformations that 

 completely modify all the reactions of coloration. This action of 

 the rays is quite different from that of heat or oxygenated water, or 

 the ordinary fixators. Newcomer (1917), and Browning and Russ 

 (1917) also irradiated plates of typhoid bacteria in a quartz spec- 

 trograph and found that the micro-organisms were killed by the in- 

 visible lethal rays ranging from 2100 A to 2900 A. 



F. L. Gates (1929, 1930, 1933) one of the most accurate of modern 

 workers, has emphasized quantitative physical methods in his 

 studies of the bactericidal action of ultraviolet rays. He showed 



