398 History of Luminescence 



glowed with a blue light when held in the ultraviolet region of a 

 solar spectrum. He wrote: " It was certainly a curious sight to see 

 the tube instantaneously light up when plunged into the invisible 

 rays; it was literally ' darkness visible.' " ^- Since the great majority 

 of fluorescent substances are excited by ultraviolet light the best 

 method of detection is to examine them in so called " dark light " 

 or " black light," an intense beam of ultraviolet wave-lengths trans- 

 mitted by a filter which absorbs the visible. Such filters are a 

 product of the twentieth century. The workers of Stokes' epoch 

 hardly dreamed of the striking fluorescent effects now obtained with 

 ultraviolet light. 



Stokes' researches were remarkably complete, including a study 

 of various light sources which might excite fluorescence. He found 

 that spark discharges were particularly effective and noted the bright 

 fluorescence of quinine during a lighting flash. He added greatly 

 to the list of fluorescent substances, using not only uranium salts 

 and aesculin, but also the red pigment of seaweeds, madder and 

 tumeric dyes, and guaiac in alcohol. He studied the bands of fluores- 

 cence spectra, worked out the relation between concentration and 

 intensity of fluorescence, and called attention to phenomena of 

 quenching, not only by addition of foreign substances but also in 

 concentrated solutions. Like Brewster, Stokes found that the fluo- 

 rescent light of solutions was unpolarized, whether the exciting light 

 was polarized or not, but that one group of substances, the platinum 

 cyanide double salts, did emit polarized light when in the form of 

 crystals, although their solutions were not even fluorescent. Through 

 his efforts fluorescence was established as a major field of inquiry. 



Research after Stokes 



METHODS OF EXCITATION 



Soon after Stokes' papers appeared, the scientific world began con- 

 firming and applying his ideas in related fields of inquiry. The term 

 fluorescence was a happy one, adopted immediately by most workers. 

 Osann (1854) confirmed the fact that blue rays which excited fluo- 

 rescence also excited phosphorescence and likewise (1857) con- 

 firmed the ability of electric sparks to excite quinine and cumarin 

 solutions. The excitation of fluorescence by flames, an inquiry of 

 Stokes, was taken up anew. Babo and J. Miiller (1856) called atten- 

 tion to the marked ability of the weak flame of carbon bisulphide 



*^ This remarkable effect was a great stimulus to the study of the ultraviolet region 

 of the spectrum. It led E. Becquerel to a " Reclamation de priority " in Cosmos 4: 509- 

 510, 1854. 



