Some Contemporary Advances in Physics — III 



By KARL K. DARROW 



ELECTROMAGNETIC waves of every frequency from 10* to 

 10^° exist; they can be generated and perceived; their frequencies 

 in nearly every instance can be measured; their actions and reactions 

 with matter can be studied. This brief statement is the synthesis 

 of a great multitude of inventions, experiments and observations upon 

 phenomena of extraordinary div-ersity and variety. When Herschel 

 in 1800 carried a thermometer across the fan-shaped beam of colored 

 light into which a sunbeam was resolved by a prism, and observed 

 that the effect of the sunbeam on the mercury column did not cease 

 when it passed beyond the red edge of the fan, he proved that the 

 boundary of the spectrum beyond the red is imposed by the limita- 

 tions of the eye and not by a deficiency of rays. Almost at the same 

 time Ritter found that the power of the violet rays to affect salts of 

 silver was shared by invisible rays beyond the violet edge of the beam. 

 Maxwell developed the notion of electromagnetic waves from his 

 theory of electricity and magnetism, and described some of the prop- 

 erties they should have; and the light-waves and the infra-red and 

 ultra-violet rays were found to have some of these properties, while 

 the outstanding discordances were explained away by Maxwell's 

 successors. Hertz and many others built apparatus for producing 

 Maxwell's waves with frequencies far below those of light, and ap- 

 paratus for detecting them, with consequences known to everyone. 

 Years after X-rays and gamma-rays were discovered emanating from 

 discharge-tubes and disintegrating atqms, Laue proved that these too 

 are waves, lying beyond the visible spectrum in the range of high 

 frequencies. Radiations emerging from collapsing atoms and radia- 

 tions diverging from wireless towers; waves conveying the solar heat 

 and waves carrying the voice; rays which disrupt atoms by extracting 

 their electrons, rays which alter atoms by rearranging their electrons, 

 rays which almost ignore atoms altogether, were successively dis- 

 covered or created; and all these radiations were brought into one 

 class, and identified with light. 



This enormously extended electromagnetic spectrum was inter- 

 rupted until lately by two regions unexplored. They were known 

 as the gap between the X-rays and the ultra-violet, and the gap be- 

 tween the infra-red and the Hertzian waves, according to the names by 

 which the various explored regions of the spectrum commonly go; 

 but to understand why they remained unclosed for so long, and what 

 kinds of rays are being found within them, it is necessary to consider 



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