208 History of Luminescence 



image of the slit. These additions completed a usable instrument, 

 and the Fraunhofer lines served as points of reference in comparing 

 one spectrum with another. Fraunhofer continued his studies in 

 1821 and 1823 and investigated the electric spark in 1824. 



In 1822-1823, John Herschel (1792-1871) had regularly used a 

 prism for studying colored flames ^^ and colored media and sug- 

 gested that such spectra might serve for detection of extremely 

 minute quantities of compounds. William Henry Fox Talbot (1800- 

 1877) emphasized the idea in 1826, and C. Wheatstone (1835) 

 studied the spectra of sparks between different metals. Spectroscopy 

 was employed more and more for examining light sources, and 

 as a means of analysis, by David Brewster " (1781-1868) and others, 

 culminating in the investigations of Gustav Robert Kirchoff ^* 

 (1824-1887) and Robert William Bunsen (1811-1899) , whose classic 

 papers, Chemische Analyze durch Spectralbeobachtung, were pub- 

 lished in 1860 and 1861. 



The earliest observations of the spectrum of inorganic phos- 

 phorescences were made in 1713 with a prism by Zanotti (1748). 

 He noted the dim monochromatic light of phosphors, but without a 

 slit and with low light intensity no true idea of spectral distribution 

 could be obtained. Priestley (1767) had no better success with elec- 

 troluminescence. When he examined the diffuse electric light in a 

 vacuum tube with a prism, it " made no sensible alteration in the 

 appearance of it." Dessaignes (1811) reported that the prism re- 

 vealed different colors when used to examine various phosphors, 

 but an accurate spectrum of inorganic luminescences could not be 

 obtained without the proper equipment. 



The first published drawings of the spectra of phosphors appear 

 to be figures 8 and 9 of plate VIII in the second volume of A. C. 

 Becquerel's Traite de Physique (Paris, 1844) . Becquerel referred 

 to the fact that he and Biot had studied the " phosphorogenic rays " 

 of the electric spark and found them different from the " luminous 

 rays "; also that E. Becquerel had studied the phosphorogenic rays 

 of the solar spectrum, which were identical with those of the electric 

 spark. The method of study was to cover paper with the powdered 

 phosphor held on with a gum, and when dry, to place the paper 

 under a solar spectrum from a slit in a dark room. His figure 8 



^^ Marggraf had used flame color tests, observed visually, to identify various salts in 

 1760. J. Herschel's work appeared in the Trans. Roy. Soc. Edinburgh 9 (II) : 445-460, 

 1823. He drew diagrams of emission spectra. 



^^ Brewster in 1832 studied absorption spectra and noted the dark absorption bands 

 of nitrogen peroxide {Trans. Roy. Soc. Edinburgh 12 (III) : 519, 1834) . 



** See G. Kirchoff, Zur Geschichte der Spectralanalyse, etc., Ann. der Physik 118: 94- 

 111, 1863. 



