!6G 



SCIENCE. 



[N. S. Vol. XXII. No. 560. 



eoult (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), determined the varia- 

 tion of velocity with the medium and wave 

 length, thus assuring to the undulatory 

 theory its ultimate triumph. Grave con- 

 cern, 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 atmos- 

 phere 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 pub- 

 lished, in particular by Kelvin (1854). 



GEOMETRIC OPTICS. 



Prior to the nineteenth century geo- 

 metric optics, having been mustered before 

 Huyghens (1690), Newton (1704), Mains 

 (1808), Lagrange (1778, 1803) and others, 

 had naturally attained a high order of de- 

 velopment. It was, nevertheless, remod- 

 eled by the great paper of Gauss (1841) 

 and was thereafter generalized step by 

 step by Listing, Mobius (1855), and par- 

 ticularly by Abbe (1872), postulating that 

 in character, the cardinal elements are in- 

 dependent 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 corre- 

 spondingly obvious fruit. The funda- 

 mental 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 accomplish- 

 ments (1838). 



The so-called astronomical refraction of 

 a medium of continuously varying index, 

 successively treated by Bouguer (1739, 

 1749), Simpson (1743), Bradley (1750, 

 1762), owes its recent refined development 

 to Bessel (1823, 1826, 1842), Ivory .(1822, 

 1823, et seq.), Radau (1884) and others. 

 Tait (1883) gave much attention to the 

 allied treatment of mirage. 



In relation to instruments the conditions 

 of aplantism were examined by Clausius 

 (1864), by Helmholtz (1874), by Abbe 

 (1873, et seq.), by Hockin (1884) and 

 others, and the apochromatic lens was in- 

 troduced by Abbe (1879). The micro- 

 scope is still well subserved by either the 

 Huyghens or the Ramsden (1873) eye- 

 piece, but the objective has undergone suc- 

 cessive stages of improvement, beginning 

 with Lister's discovery in 1830. Amici 

 (1840) introduced the principle of immer- 

 sion; Stephenson (1878) and Abbe (1879), 

 homogeneous 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 construction to a degree of astonish- 

 ing excellence. 



SPECTRUM. DISPERSION. 



Curiously, the acumen of Newton (1866, 



