PROGRESS IN NINETEENTH CENTURY 49 



The electromagnetic theory lends itself particularly well to the same 

 phenomena, and Koldzek (1887, 1888), Goldhammer (1892), Helm- 

 holtz (1892), Drude (1893), and others instanced its adaptation with 

 success. 



Photometry, Fluorescence, Photochemistry 



The cosine law of Lambert (1760) has since been interpreted in 

 a way satisfying modern requirements by Fourier (1817, 1824) and 

 by Lommel (1880). Among new resources for the experimentalist 

 the spectrophotometer, the Lummer-Brodhun photometer (1889), 

 and Rood's flicker photometer (1893, 1899), should be mentioned. 



Fluorescence, though ingeniously treated by Herschel (1845, 

 1853) and Brewster (1846, et seq.), was virtually created in its philo- 

 sophical aspects by Stokes in his great papers (1852, et seq.) on the 

 subject. In recent years Lommel (1877) made noteworthy contribu- 

 tions. Phosphorescence has engaged the attention of E. Becquerel 

 (1859), among others. 



The laws of photochemistry are in large measure due to Bunsen 

 and Roscoe (1857, 1862). The practical development of photography 

 from its beginnings with Daguerre (1829, 1838) and Niepce and 

 Fox-Talbot (1839), to its final improvement by Maddox (1871) 

 with the introduction of the dry plate, is familiar to all. Vogel's 

 (1873) discovery of appropriate sensitizers for different colors has 

 added new resources to the already invaluable application of photo- 

 graphy to spectroscopy. 



Interference 



The colors of thin plates treated successively by Boyle (1663), 

 Hooke (1665), and more particularly by Newton (1672, Optiks, 

 1704), became in the hands of Young (1802) the means of framing 

 an adequate theory of light. Young also discovered the colors of 

 mixed plates and was cognizant of loss of half a wave-length on 

 reflection from the denser medium. Fresnel (1815) gave an inde- 

 pendent explanation of Newton's colors in terms of interference, 

 devising for further evidence his double mirrors (1816), his biprism 

 (1819), and eventually the triple mirror (1820). Billet's plates and 

 split lens (1858) belong to the same classical order, as do also Lloyd's 

 (1837) and Haidinger's (1849) interferences. Brewster's (1817) 

 observation of interference in case of thick plates culminated in 

 the hands of Jamin (1856, 1857) in the useful interferometer. The 

 scope of this apparatus was immensely advanced by the famous 

 device of Michelson (1881, 1882), which has now become a funda- 

 mental instrument of research. Michelson's determination of the 

 length of the meter in terms of the wave-length of light with as- 

 tounding accuracy is a mere example of its accomplishments. 



