Fluorescence 409 



molecules has led to an ever increasing interest in the subject, with 

 the publication of a large selection of twentieth-century books, from 

 which the latest theories on fluorescence may be obtained (see Chap- 

 ter VIII for nineteenth-century theories) . The early observations 

 on lignum nephriticum extract, the later recognition of an unusual 

 play of colors in other solutions, and the penetrating research of 

 Stokes have led to an important topic in the physical study of light, 

 a topic well represented in every modern textbook of physics. 



