L. Page — A Century's Progress in Physics. 321 



elementary charge. Combining this result with the 



value of — found by Thomson, the mass of the electron 



comes out to be about one eighteen-hundredth that of an 

 atom of the lightest known element, hydrogen. 



That the electron is a fundamental constituent of all 

 matter is attested by the fact that charge and mass are 

 the same regardless of the source or manner of produc- 

 tion. Whether emitted by a heated metal, under the 

 action of ultra-violet light, from a radioactive substance, 

 by a body exposed to X-rays, as a result of friction, it is 

 the same negatively charged particle that constitutes the 

 cathode ray of the discharge tube. Moreover, it makes 

 its effect felt indirectly in many other phenomena, and 

 from an investigation of some of these the ratio of 

 charge to mass, can be determined independently. Of 

 such perhaps the most interesting is the 'Zeeman effect. 



Spectroscopy. — Early in the nineteenth century Fraun- 

 hofer had observed that the solar spectrum is crossed 

 by a large number of dark lines. Their presence was 

 unexplained until in 1859 Kirchhoff and Bunsen showed 

 ' ' that a colored flame, the spectrum of which contains 

 bright sharp lines, so weakens rays of the color of these 

 lines when they pass through it, that dark lines appear 

 in place of bright lines as soon as there is placed behind 

 the flame a light of sufficient intensity, in which the lines 

 are otherwise absent. " For intra-atomic oscillators 

 must have the natural frequency of the radiation which 

 they emit, and consequently resonance will take place 

 when they are exposed to rays of this frequency coming 

 from an outside source, and selective absorption ensue. 

 By comparing the bright lines in the spectra of metallic 

 vapors made luminous by a gas flame with the dark lines 

 in the sun's spectrum these investigators showed that 

 many of the common terrestrial elements exist in the 

 sun. The interest in spectroscopy grew rapidly. The 

 excellent diffraction gratings made by Eutherfurd were 

 succeeded by the superior concave gratings of Rowland. 

 In 1877 Draper (14, 89, 1877) announced the discovery of 

 the bright lines of oxygen in the solar spectrum, but his 

 interpretation of his photographs has not been corrob- 

 orated by the work of later investigators. Langley (11, 

 401, 1901), by the aid of his newly invented bolometer, 

 succeeded in detecting the emission of energy from the 



Am. Jour. Sci —Fourth Series, Vol. XL VI, No. 271.— July, 1918. 

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