The Nineteenth Century 207 



research of Edmond Becquerel on phosphorescence of solids and 

 the equally comprehensive investigation of G. G. Stokes on fluores- 

 cence of solutions, while electroluminescence of gases was studied by 

 J. Pliicker, Wm. Crookes, and many others. 



During the century, three new types of luminescence were identi- 

 fied, fluorescence in 1852 by G. G. Stokes, radioluminescence from 

 bombardment with new kinds of rays in 1858 by J. Pliicker, and 

 chemiluminescence of organic solutions in 1877 by B. Radziszewski. 

 Only a brief statement on these new luminescences, and on impor- 

 tant monographs and textbooks can be given in subsequent sections. 

 Details of the research on various luminescences will be found in 

 corresponding chapters. 



THE RISE OF SPECTROSCOPY 



The trend of luminescence study at the beginning of the eight- 

 eenth-century had been concerned with the discovery of new phos- 

 phors and the realization that almost all bodies (except metals 

 and water) could be excited to luminesce in some way. Electrical 

 knowledge had brought the ability to produce high voltages and 

 powerful electrical discharges. At the beginning of the nineteenth 

 century, the most important factor in the advance of luminescence 

 knowledge was the extension of the solar spectrum into the infrared 

 and ultraviolet regions, cotipled with the rise of spectroscopy ^° as a 

 tool for the study of matter. 



Spectral study during the nineteenth century was made possible 

 by the construction of a real spectroscope. The first necessity was 

 the introduction of a slit in front of the light source, a procedure 

 standardized by Wollaston in 1802. Previously, in 1752, the Scots- 

 man, Thomas Melvill (1726-1753), had examined (1756) a spirit 

 flame containing nitre and sea salt, using a pinhole diaphragm and 

 prism. He remarked that " the bright yellow which prevails so 

 much over other colours, must be of one determined degree of re- 

 frangibility." However, the pinhole was soon forgotten, and it was 

 the slit which brought out the dark lines in sunlight for Wollaston ^^ 

 in 1802. They were later rediscovered by Fraunhofer (1814), who 

 had added the collimating tube and a telescope for observing the 



^^ See H. Kayser, Handbuch der Spectroscopic 1: 3-130, Chap. I, Geschichte der Spec- 

 troscopie, Leipzig, 1900. 



^^ Wm. Hyde Wollaston first saw dark (Fraunhofer) lines in the sun's spectrum 

 (Phil. Trayis. 92: 378, 1802) , but merely pictured the dark lines as separating the 

 various colors. They were overlooked by Newton because of a poor prism and because 

 his light came from a round hole instead of a very narrow slit. Joseph Fraunhofer's 

 (1787-1826) description was published in 1814. He determined their wave-lengths in 

 1821 and 1823. 



