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BELL SYSTEM TECHNICAL JOURNAL 



arc, which probably belong to the second or third class, but it is not 

 certain which. If we gather all these classes together into a single 

 great class of natural rays, extending from .02A or 2.10""' cm. to 

 4,00(),000A or 0.04 cm., the\- may be contrasted with the artificial 

 rays generated by man-made electrical circuits, King entireh" beyond 

 the long-wave limit of their range. ^ 



One of the two lacimae in the spectrum, extending from 0.4 mm. to 

 7 mm., separated the range of natural rays from the range of artificial 

 rays. To close this gap it was necessary literally to in\enl new rays, 

 by designing oscillating electrical circuits which would generate fre- 

 quencies which perhaps had ne\'er existed before in nature. The other 

 lacuna, extending from 13A to 1200A, lay by contrast in the very 

 centre of the range of natural rays, and precisely where we expect to 

 find the frequencies resulting from certain peculiarly interesting and 

 important processes in the electron-systems of atoms. These pro- 

 cesses, it appears, are not in all cases easy to incite by the usual 

 methods of stimulating atoms to radiate; but this difficulty is only one, 

 and probably the least serious one, of the three hindrances which com- 

 bined to delay the exploration of this region. A second impediment 

 comes from the limitations of our devices for measuring wa\o-Ienglh, 

 every one of which is una\'ailable over a certain sector of the region, 

 extending roughly from 13A to loOA (limits which may later be forced 

 somewhat closer together) ; but the most conspicuous obstacle is 

 the extraordinary obstructiveness and opacity of e\ery kind of matter 

 to these rays. 



The ability of eleclroniagnctic wa\es to penetrate matter varies 

 enormously from one part of the spectrum to another. At the upper- 

 most end of the frequency-scale, the rays penetrate every sort of 

 matter with astonishing ease. A layer of lead 8 mm. thick is required 

 to remove half of the energy of a ray of wavelength .025A; and even 

 this, it is probable, is not absorbed in the strict sense of lieing con- 

 verted from radiant energy into another form, being merely deffecled 

 or scattered out of its original direction of motion.'' With rays of 

 greater wave-length, a true absorption is superposed upon the scatter- 

 ing, and increases very rapidh', about as the third power of the wave- 

 length. The absorbed energ\- is used in extracting electrons from 



' The distinction between natural and artificial rays is striking, but I fear not 

 quite exact, since lightning-discharges and the causes of "static" offer instances of 

 natural sources of radio fre<|uencies. Also the selective absorptions of certain 

 substances in the Hertzian range strongly suggest natural emission-frequencies. 

 Still the distinction is not yet unsound enough to l)e dangerous. 



' If A. n. Compton's theory of X-ray scattering is eventually triumphant, it will 

 be neces.sary to admit that some radiant energ>'' is transformed into kinetic energy 

 of moving mas-ses when scattering occurs. 



