50 PHYSICS 



Wiener (1890) in his discovery of the stationary light-wave intro- 

 duced an entirely new interference phenomenon. The method was 

 successfully applied to color photography by Lippmann (1891, 

 1892), showing that the electric and not the magnetic vector is 

 photographically active. 



The theory of interferences from a broader point of view, and 

 including the occurrence of multiple reflections, was successively 

 perfected by Poisson (1823), Fresnel (1823), Airy (1831). It has 

 recently been further advanced by Feussner (1880, et seq.}, Sohncke 

 and Wangerin (1881, 1883), Rayleigh (1889), and others. The inter- 

 ferences along a caustic were treated by Airy (1836), but the 

 endeavor to reconstruct geometric optics on a diffraction basis 

 has as yet only succeeded in certain important instances, as already 

 mentioned. 



Diffraction 



Though diffraction dates back to Grimaldi (1665) and was well 

 known to Newton (1704), the first correct though crude interpret- 

 ation of the phenomenon is due to Young (1802, 1804). Independ- 

 ently Fresnel (1815) in his original work devised similar explanations, 

 but later (1818, 1819, 1826) gave a more rational theory in terms 

 of Huyghens's principle, which he was the first adequately to inter- 

 pret. Fresnel showed that all points of a wave-front are concerned 

 in producing diffraction, though the ultimate critical analysis was 

 left to Stokes (1849). 



In 1822 Fraunhofer published his remarkable paper, in which, 

 among other inventions, he introduced the grating into science. 

 Zone plates were studied by Cornu (1875) and by Soret (1875). 

 Rowland's concave grating appeared in 1881 ; Michelson's echelon 

 spectrometer in 1899. 



The theory of gratings and other diffraction phenomena was 

 exhaustively treated by Schwerd (1837). Babinet established the 

 principle bearing his name in 1837. Subsequent developments were 

 in part concerned with the improvement of Fresnel's method of 

 computation, in part with a more rigorous treatment of the theory 

 of diffraction. Stokes (1850, 1852) gave the first account of the 

 polarization accompanying diffraction, and thereafter Rayleigh 

 (1871) and many others, including Kirchhoff (1882, 1883), profoundly 

 modified the classic treatment. Airy (1834, 1838) and others elabor- 

 ately 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 scat- 

 tering propounded by Rayleigh (1871) in his dynamics of the blue 

 sky. This great theory which Rayleigh has repeatedly improved 

 (1881, et seq.) has since superseded all other relevant explanations. 



