Phosphorescence 355 



strument of 1858, which solved the problem for times as short as 

 10"* seconds^® is well known. The material to be examined is 

 mounted between two revolving disks each containing a number 

 of staggered holes near the periphery, so arranged that light passing 

 through a hole in the first disk and striking the material cannot be 

 seen by looking through a hole in the second disk, A small fraction 

 of a second later, depending on rate of revolution of the disks, the 

 exciting light is cut off from the material and the hole in the second 

 disk has moved into a position where observation becomes possible. 



Becquerel's experiments with this instrument showed that a great 

 many compounds phosphoresced for varying lengths of time after 

 illumination, times measured either in ten thousands of a second or 

 in seconds. The distinction between fluorescence and phosphores- 

 cence appears to break down. However, Becquerel was never able 

 to measure the duration of fluorescence in solutions,''^ which was 

 later found to be of the order of a one-hundred millionths of a 

 second. The expression " true fluorescence " is sometimes reserved 

 for such light emissions, but the observation that fluorescence in 

 solution becomes a phosphorescence if the solution gels (Wiede- 

 mann, 1887) or at liquid air temperatures (Dewar, 1894) , makes 

 the distinction rather arbitrary.^^ In this book, phosphorescence and 

 fluorescence have been separated as chapters, purely because, dur- 

 ing most of the long history of the subjects, the two were not gen- 

 erally recognized as similar phenomena. 



Becquerel was never willing to admit a difference between phos- 

 phorescence and fluorescence except one of time, although he did 

 describe substances observed in his phosphoroscope in which the 

 color of the light at the first moment of exposure differed from the 

 later color. On the other hand Stokes (1852) claimed that, in addi- 

 tion to the fact that a phosphorescence always had duration, the 

 phosphorescent light from material spread in a thin film and sharply 

 illuminated actually spread sideways, whereas fluorescent light did 

 not. He also argued that phosphorescence and fluorescence were 

 quite distinct because different substances exhibited these types of 

 luminescence. 



'^ Time intervals of 10"* seconds can now be measured. 



«'' Becquerel was unable to observe an afterglow in quartz, sulphur, phosphorus, 

 metals, or liquids. Even after Wiedemann (1888) improved the phosphoroscope to 

 the point where a few millionths of a second could be measured, a phosphorescence 

 of solutions could not be detected. 



^^ Modern theory indicates that duration of the light is not necessarily a distinc- 

 tion between phosphorescence and fluorescence. A better test is to determine whether 

 the excited state is paramagnetic, when phosphorescence is indicated. If both phos- 

 phorescence and fluorescence excited states occur in one molecule, the phospho- 

 rescence emission (observed at low temperature) is of longer wave-length and of 

 longer duration. 



