The Nineteenth Century 219 



phase of the subject and is considered in appropriate chapters, chiefly 

 in Chapter VIII. His book is illustrated with four beautiful colored 

 plates of spectra, one of the sun, a second of various flames, and two 

 more of various phosphorescences and fluorescences. The wave- 

 length scale is calibrated by Fraunhofer lines rather than micra. 

 In contrast to his father, E. Becquerel presented no universal theory 

 to account for the light emission but contented himself with the 

 collection of new quantitative data regarding the light. 



luminescence after 1870 



The book of E. Becquerel, La Lumiere, published in 1867, marks 

 the end of an epoch in study of phosphorescence. The work of 

 Stokes (1852) on fluorescence and that of Julius Plucker (1801- 

 1868) on electroluminescence (1858) were to lead to the rapid 

 development of these branches of luminescence study and to the 

 all important phenomena of radioluminescence. Among the many 

 men who followed and made luminescence a major interest, the 

 names of Wm. Crookes of England, P. E. Lecoq de Boisbaudran 

 (1838-1912) of France, and Eugene Lommel (1837-1899) , Eilhardt 

 Wiedemann (1852-1928) , and Philipp Lenard (1862-1947) of Ger- 

 many take precedence. In the United States during this period 

 Joseph Henry (1799-1878) and John William Draper (1811-1882), 

 both members of the American Philosophical Society, did more to 

 foster experiment than others, but the great surge of luminescence 

 research in America was to come in the next century with E. L. 

 Nichols (1854-1937), E. Merritt (1865-1948) and R. W. Wood 

 (1868-1955) leading the way. 



In devoted study of luminescences of all kinds, the Wiedemann 

 family of Germany is the counterpart of the Becquerel family of 

 France. The father of Eilhardt Wiedemann, Gustav Heinrich Wiede- 

 mann (1826-1899) , professor of physics at the University of Leipzig 

 since 1871, was interested in electricity and magnetism and pub- 

 lished only a few papers on luminescence (1876) . He edited the 

 Annalen der Physik und Chemie from 1877 until his death in 1899. 

 During the last six years he was aided by his son. His wife, Clara 

 Laura Wiedemann, one of the Mitscherlich family and also a physi- 

 cist, translated the works of Tyndall and wrote on light and sound. 

 The son, Eilhardt (Ernst Gustav) Wiedemann, obtained his Ph.D. 

 at Leipzig in 1872, became a professor there and then (1886) was 

 appointed professor of physics at the University of Erlangen. His 

 early work had to do with polarization and refraction of light, later 

 with the specific heat of gases, finally with their spectra in electrical 



