48 PHYSICS 



elements contained in the sun (1891), each marked a definite stage 

 of advance of the subject. Mitscherlich (1862, 1863) probably was 

 the first to recognize the banded or channeled spectra of compound 

 bodies. Balmer (1885) constructed a valuable equation for recog- 

 nizing the distribution of single types of lines. Kayser and Runge 

 (1887, et seq.) successfully analyzed the structure of the spectra of 

 alkaline and other elements. 



The modernized theory of the grating had been given by Rayleigh 

 in 1874 and was extended to the concave grating by Rowland (1892, 

 1893) and others. A general theory of the resolving power of pris- 

 matic systems is also due to Rayleigh (1879, 1880), and another to 

 Thollon (1881). 



The work of Rowland for the visible spectrum was ably paral- 

 leled by Langley's investigations (1883 et seq.) of the infra-red, dating 

 from the invention of the bolometer (1881). Superseding the work 

 of earlier investigators like Fizeau and Foucault (1878) and others, 

 Langley extended the spectrum with detailed accuracy to over 

 eight times its visible length. The solar and the lunar spectrum, the 

 radiations of incandescent and of hot bodies, were all specified abso- 

 lutely and with precision. With artificial spectra Rubens (1892, 

 1899) has since gone further, reaching the longest heat-waves known. 



A similarly remarkable extension was added for the ultra-violet 

 by Schumann (1890, 1892), contending successfully with the grad- 

 ually increasing opacity of all known media. 



Experimentally the suggestion of the spectroheliograph by Lock- 

 yer (1868) and by Janssen (1868) and its brilliant achievement by 

 Hale (1892) promise notable additions to our knowledge of solar 

 activity. 



Finally, the refractions of absorbing media have been of great 

 importance in their bearing on theory. The peculiarities of metallic 

 reflection were announced from his earlier experiments (1811) by 

 Arago in 1817 and more fully investigated by Brewster (1815, 1830, 

 1831). F. Neumann (1832) and MacCullagh (1837) gave sharper 

 statements to these phenomena. Equations were advanced by 

 Cauchy (1836, et seq.) for isotropic bodies, and later with greater 

 detail by Rayleigh (1872), Ketteler (1875, et seq.), Drude (1887, et 

 seq.), and others. Jamin (1847, 1848) devised the first experiments of 

 requisite precision and found them in close agreement with Cauchy 's 

 theory. Kundt (1888) more recently investigated the refraction of 

 metallic prisms. 



Anomalous dispersion was discovered by Christiansen in 1870, 

 and studied by Kundt (1871, et seq.). Sellmeyer's (1872) powerful 

 and flexible theory of dispersion was extended to include absorp- 

 tion effects by Helmholtz (1874), with greater detail by Ketteler 

 (1879, et seq.), and from a different point of view by Kelvin (1885). 



