Septembeb 22, 1905.] 



SCIENCE. 



367 



1704) stopped short of the ultimate condi- 

 tions of purity of spectrum. It was left 

 to Wollaston (1802), about one hundred 

 years later, to introduce the slit and ob- 

 serve the dark lines of the solar spectrum. 

 Fraunhofer (1814, 1815, 1823) mapped 

 them out carefully and insisted on their 

 solar origin. Brewster (1833, 1834), who 

 afterwards (1860) published a map of 

 3,000 lines, was the first to lay stress on the 

 occurrence of absorption, believing it to be 

 atmospheric. Forbes (1836) gave even 

 greater definiteness to absorption by refer- 

 ring it to solar origin. Foucault (1849) 

 pointed out the coincidence of the sodium 

 lines with the D group of Fraunhofer, and 

 discovered the reversing effect of sodium 

 vapor. A statement of the parallelism of 

 emission and absorption came from Ang- 

 strom (1855) and with greater definiteness 

 and ingenious experiments from Stewart 

 (1860). Nevertheless, it was reserved to 

 Kirchhoff and Bunsen (1860, 1861) to give 

 the clear-cut distinctions between the con- 

 tinuous spectra and the characteristically 

 fixed bright-line or dark-line spectra upon 

 which spectrum analysis depends. Kirch- 

 hoff 's law was announced in 1861 and the 

 same year brought his map of the solar 

 spectrum and a discussion of the chemical 

 composition of the sun. Huggins (1864, 

 et seq.). Angstrom (1868), Thalen (1875), 

 followed with improved observations on the 

 distribution and wave-length of the solar 

 lines; but the work of these and other ob- 

 servers was suddenly overshadowed by the 

 marvelous possibilities of the Rowland con- 

 cave grating (1882, et seq.). Rowland's 

 maps and tables of the solar spectrum as 

 they appeared in 1887, 1889, et seq., his 

 summary of the 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) con- 

 structed a valuable equation for recogniz- 

 ing the distribution of single types of lines. 

 Kayser and Runge (1887, et seq.) success- 

 fully 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 gen- 

 eral 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 paralleled by Langley's 

 investigations (1883, et seq.) of the infra- 

 red, dating from the invention of the bo- 

 lometer (1881). Superseding the work of 

 earlier investigators like Fizeau and Fou- 

 cault (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 absolutely 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 gradually increasing opacity of all 

 known media. 



Experimentally the suggestion of the 

 spectroheliograph by Lockyer (1868) and 

 by Janssen (1868) and its brilliant achieve- 

 ment by Hale (1892) promise notable ad- 

 ditions 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 



