The Nineteenth Century 209 



shows that calcium sulphide has two separate luminescent bands, 

 one in the violet and one in the ultraviolet, whereas the barium 

 sulphide of figure 9 has a single wider band extending into both 

 violet and ultraviolet. For comparison, a solar spectrum, with the 

 Fraunhofer lines was also reproduced. ^^ 



Later advance in knowledge of spectral composition of the light 

 of phosphors is due to the systematic researches of Edmond Bec- 

 querel. It is true that fluorescence spectra ^® had been observed 

 previously, and Stokes had examined the various fluorescences care- 

 fully, noting the series of emission bands of uranium compounds 

 and other substances. In fact he announced in 1852 what came to be 

 known as Stokes' law that the emitted light is always of longer wave- 

 length than the exciting light, but published no adequate plates of 

 fluorescent spectra. 



E. Becquerel's earlier papers (1843, 1848, 1858) were mostly con- 

 cerned with the composition of the exciting light, but in his 1859 

 contribution, he used a spectroscope of the modern type. The paper 

 is illustrated by a plate showing the phosphorescence emission bands 

 in fifteen different solids. His wave-length scale was in relation to 

 Fraunhofer lines. Accurate investigation of the spectral charac- 

 teristic of the luminescent light of solids had begun. 



Although colored lines had been noted in the spark spectra of 

 mercury vapor and other metals as early as 1835 by C. Wheatstone, 

 and David Alter, an American M. D., had described the line spec- 

 trum of hydrogen and other gases in 1855, nevertheless, accurate 

 electroluminescent spectra of gases will always be associated with 

 the name of Julius Pliicker (1801-1868), whose dumbbell-shaped 

 tubes containing gases at low pressure are still used for spectroscopic 

 observation. The glass capillary connecting the two bulbs with 

 sealed-in platinium electrodes allow an intensity of electrolumines- 

 cence quite adequate for spectral examination. Pliicker's observa- 

 tions were published in 1859, about the same time Becquerel's dis- 

 coveries were announced to the scientific world, but plates " of his 

 gas spectra did not appear until the Pliicker and Hittorf paper in 

 1865. The work of these men not only paved the way for detection 

 of new methods of exciting luminescence, by electrons, by beams of 



"Although Fraunhofer had detemiined the wave-lengths of his lines in 1821 and 

 1823, the wave-length scale was not generally adopted until after the independent 

 measurements of J. Muller, of E. Mascart, and of A. J. Angstrom, all in 1863. Com- 

 parison of spectra was made by Fraunhofer lines. 



^« Brewster (1833, 1846) merely described the color of fluorescent light and Herschel 

 (1845) noted that in the blue light of fluorspar when examined with a prism, red, 

 orange, and yellow were absent or deficient but some green was present. 



^^V. S. M. van der Willigen (1858) published a rather poor plate of gas spectra in 

 his Dutch paper. 



