PROGRESS IN NINETEENTH CENTURY 47 



Radau (1884), and others. Tait (1883) gave much attention to the 

 allied treatment of mirage. 



In relation to instruments the conditions of aplantism were exam- 

 ined by Clausius (1864), by Helmholtz (1874), by Abbe (1873, et 

 seq.), by Hockin (1884), and others, and the apochromatic lens was 

 introduced by Abbe (1879). The microscope is still well subserved 

 by either the Huyghens or the Ramsden (1873) eye-piece, but the 

 objective has undergone successive stages of improvement, begin- 

 ning with Lister's discovery in 1830. Amici (1840) introduced the 

 principle of immersion; Stephenson (1878) and Abbe (1879), homo- 

 geneous immersion; and the Abbe-Zeiss apochromatic objective 

 (1886), the outcome of the Jena-glass experiments, marks, perhaps, 

 the high-water mark of the art for the microscope. Steinheil (1865, 

 1866) introduced the guiding principle for photographic objectives. 

 Alvan Clark carried the difficult technique of telescope lens con- 

 struction to a degree of astonishing excellence. 



Spectrum Dispersion 



Curiously, the acumen of Newton (1666, 1704) stopped short of 

 the ultimate conditions of purity of spectrum. It was left to Wollas- 

 ton (1802), about one hundred years later, to introduce the slit 

 and observe 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 3000 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 referring 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 Angstrom (1855) and with greater defin- 

 iteness and ingenious experiments from Stewart (1860). Never- 

 theless, it was reserved to Kirchhoff and Bunsen (1860, 1861) to 

 give the clear-cut distinctions between the continuous spectra and 

 the characteristically fixed bright-line or dark-line spectra upon 

 which spectrum analysis depends. Kirchhoff'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 im- 

 proved observations on the distribution and wave-length of the solar 

 lines; but the work of these and other observers was suddenly over- 

 shadowed by the marvelous possibilities of the Rowland concave 

 grating (1882, et seq.). Rowland's maps and tables of the solar spec- 

 trum as they appeared in 1887, 1889, et seq., his summary of the 



