II. THE STRUCTURE OF FLUORESCENCE SPECTRA. 



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A fluorescence spectrum consists of one or more bright bands, and 

 these may greatly vary in width, from the very broad bands, filling a 

 great part of the visible spectrum, characteristic of the fluorescent 

 dyestuffs and the phosphorescent sulphides, to the line-like bands of 

 the ruby. 



Such a spectrum is either a homogeneous complex of systematically 

 related components or a heterogeneous complex of unrelated compo- 

 nents. In either case the components frequently overlap, giving the 

 appearance of a single band, which may be described as a mixed band 

 (an unresolved heterogeneous complex) or a homogeneous band, respec- 

 tively. Where the components overlap less completely or not at all the 

 appearance is that of a group of bands. 



It is probable that a heterogeneous complex is always the result of 

 a mixture of two or more compounds the fluorescence of each of which 

 by itself gives a homogeneous 

 complex. 



The phosphorescent sul- 

 phides afford spectra which 

 may serve to illustrate the 

 above classification. A stron- 

 tium sulphide with bismuth as 

 the active metal and a flux of 

 sodium sulphate, for example, 

 has a fluorescence spectrum 

 which appears to the eye to 

 consist of a single band with 

 its crest at 0.480 /z. A recent 

 spectrophotometric explora- 

 tion by Dr. H. L. Howes, 1 

 however, shows a group of 

 closely over-lapping compo- 

 nents (see fig. 2) . The crests 

 of these are located as shown 

 in table 1; and as they are 

 systematically related, form- 

 ing members of a series having 

 a uniform interval of frequency ' 0)a 



difference, this is to be regarded 

 as a homogeneous band or homogeneous complex. 



Similarly, the fluorescence of a barium sulphide with copper as the 

 active metal and a flux of sodium borate, when viewed through an 



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1 Proceedings American Philosophical Society, LVI, p. 258. 1917. 



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