PROGRESS IN NINETEENTH CENTURY 51 



Polarization 



An infinite variety of polarization phenomena grew out of Bar- 

 tholinus's (1670) discovery. Sound beginnings of a theory were 

 laid by Huyghens (Traite, 1690), whose wavelet principle and ele- 

 mentary 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 

 Malus (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 con- 

 nection with parallel plane-polarized light, phenomena which under 

 the observations of Arago and Fresnel (1816, 1819), Biot (1816), 

 Brewster (1813, 1814, 1818), and others grew immensely 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 transversality of light-waves, offering proof which was sub- 

 sequently made rigorous by Verdet (1850). Though a tentative 

 explanation was here again given by Young (1814), the first ade- 

 quate theory of the behavior of thin plates of aeolotropic media 

 with polarized light came from Fresnel (1821). 



Airy (1833) elucidated a special case of the gorgeously compli- 

 cated interferences obtained with convergent pencils; Neumann 

 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 verified 

 by many physicists, notably by Fizeau (1862) showing interferences 

 for path differences 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 gen- 

 eralized elementary wave-surface, and the correctness of his explan- 

 ation has since been substantiated by a host of observers. Stokes 

 (1862, et seq.} was unremittingly active in pointing out the theoret- 

 ical bearing 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 polarization, discovered by Arago 

 (1811) and enlarged by Biot (1815), has recently been placed in 



