386 



SCIENCE. 



[N. S. Vol. XXII. No. 561. 



Kirchhoff (1882, 1883), profoundly modi- 

 fied the classic treatment. Airy (1834, 

 1838) and others elaborately examined the 

 diffraction due to a point source in view 

 of its important bearing on the efficiency 

 of optical instruments. 



A unique development of diffraction is 

 the phenomenon of scattering propounded 

 by Rayleigh (1871) in his dynamics of the 

 blue sky. This great theory which Eay- 

 leigh has repeatedly improved (1881, et 

 seq.) has since superseded all other rele- 

 vant explanations. 



POLARIZATION. 



An infinite variety of polarization phe- 

 nomena grew out of Bartholinus 's (1670) 

 discovery. Sound beginnings of a theory 

 were laid by Huyghens ('Traite,' 1690), 

 whose wavelet principle and elementary 

 wave front have persisted as an invaluable 

 acquisition, to be generalized by Fresnel in 

 1821. 



Fresh foundations in this department of 

 optics were laid by Mains (1810) in his 

 discovery of the cosine law and the further 

 discovery of the polarization of reflected 

 light. Later (1815) Brewster adduced the 

 conditions of maximum polarization for 

 this case. 



In 1811 Arago announced the occurrence 

 of interferences in connection with paral- 

 lel plane-polarized light, phenomena which 

 under the observations of Arago and Fres- 

 nel (1816, 1819), Biot (1816), Brewster 

 (1813, 1814, 1818) and others grew im- 

 mensely in variety, and in the importance 

 of their bearing on the undulatory theory. 

 It is on the basis of these phenomena that 

 Fresnel in 1819 insisted on the transver- 

 sality of light waves, offering proof which 

 was subsequently made rigorous by Verdet 

 (1850). Though a tentative explanation 

 was here again given by Young (1814), 

 the first adequate theory of the behavior of 



thin plates of aeolotropic media with polar- 

 ized light came from Fresnel (1821). 



Airy (1833) elucidated a special case of 

 the gorgeously complicated interferences 

 obtained with convergent pencils; Neu- 

 mann in 1834 gave the general theory. 

 The forbidding equations resulting were 

 geometrically interpreted by Bertin (1861, 

 1884), and Lommel (1883) and Neumann 

 (1841) added a theory for stressed media, 

 afterwards improved by Pockels (1889). 



The peculiarly undulatory character of 

 natural light owes its explanation largely 

 to Stokes (1852), and his views were veri- 

 fied by many physicists, notably by Fizeau 

 (1862) showing interferences for path dif- 

 ferences of 50,000 wave-lengths and by 

 Michelson for much larger path differences. 



The occurrence of double refraction in 

 all non-regular crystals was recognized by 

 Haiiy (1788) and studied by Brewster 

 (1818). In 1821, largely by a feat of 

 intuition, Fresnel introduced his general- 

 ized elementary wave surface, and the cor- 

 rectness of his explanation has since been 

 substantiated by a host of observers. 

 Stokes (1862, et seq.) was unremittingly 

 active in pointing out the theoretical bear- 

 ing of the results obtained. Hamilton 

 (1832) supplied a remarkable criterion of 

 the truth of Fresnel's theory deductively, 

 in the prediction of both types of conic 

 refraction. The phenomena were detected 

 experimentally by Lloyd (1833). 



The domain of natural rotary polariza- 

 tion, discovered by Arago (1811) and en- 

 larged by Biot (1815), has recently been 

 placed in close relation to non-symmetrical 

 chemical structure by LeBel (1874) and 

 van't Hoff (1875), and a tentative molecu- 

 lar theory was advanced by Sohncke 

 (1876). 



Boussinesq (1868) adapted Cauchy's 

 theory (1842) to these phenomena. Inde- 

 pendent elastic theories were propounded 



