46 PHYSICS 



had worked as long ago as 1675, and Bradley in 1727. It remained 

 to actually measure this enormous velocity in the laboratory, appar- 

 ently an extraordinary feat, but accomplished simultaneously by 

 Fizeau (1849) and by the aid of Wheatstone's revolving mirror (1834) 

 by Foucault (1849, 1850, 1862). Since that time precision has been 

 given to this important constant by Cornu (1871, 1873, 1874), Forbes 

 and Young (1882), Michelson (1878, et seq.}, and Newcomb (1885). 

 Foucault (1850), and more accurately Michelson (1884), deter- 

 mined the variation of velocity with the medium and wave-length, 

 thus assuring to the undulatory theory its ultimate triumph. Grave 

 concern, however, still exists, inasmuch as Michelson and Morley 

 (1886) by the most refined measurement, and differing from the 

 older observations of Fizeau (1851, 1859), were unable to detect 

 the optical effect of the relative motion of the atmosphere and the 

 luminiferous ether predicted by theory. 



Romer's observation may in some degree be considered as an 

 anticipation of the principle first clearly stated by Doppler (1842), 

 which has since become invaluable in spectroscopy. Estimates of 

 the density of the luminiferous ether have been published, in par- 

 ticular by Kelvin (1854). 



Geometric optics 



Prior to the nineteenth century geometric optics, having been 

 mustered before Huyghens (1690), Newton (1704), Malus (1808), 

 Lagrange (1778, 1803), and others, had naturally attained a high 

 order of development. It was, nevertheless, remodeled by the great 

 paper of Gauss (1841), and was thereafter generalized step by step 

 by Listing, Mobius (1855), and particularly by Abbe (1872), post- 

 ulating that in character, the cardinal elements are independent 

 of the physical reasons by which one region is imaged in another. 



So many able thinkers, like Airy (1827), Maxwell (1856, et seq.}, 

 Bessel (1840, 1841), Helmholtz (1856, 1867), Ferraris (1877, 1880), 

 and others have contributed to the furtherance of geometric optics, 

 that definite mention is impossible. In other cases, again, profound 

 methods like those of Hamilton (1828, et seq.}, Kummer (1859), 

 do not seem to have borne correspondingly obvious fruit. The fun- 

 damental bearing of diffraction on geometric optics was first pointed 

 out by Airy (1838), but developed by Abbe (1873), and after him by 

 Rayleigh (1879). An adequate theory of the rainbow, due to Airy 

 and others, is one of its picturesque accomplishments (1838). 



The so-called astronomical refraction of a medium of continu- 

 ously varying index, successively treated by Bouguer (1739, 1749), 

 Simpson (1743), Bradley (1750, 1762), owes its recent refined de- 

 velopment to Bessel (1823, 1826, 1842), Ivory (1822, 1823, et seq.), 



