PREFACE. 



This volume, the completion of which has been much delayed by the 

 participation of America in the World War, contains the results of an 

 investigation covering a period of eight years. The discovery by 

 Becquerel and Onnes, that the fluorescence of certain uranyl compounds 

 is resolved into groups of narrow line-like bands when these substances 

 are excited to luminescence at very low temperatures, suggested to the 

 present authors the desirability of a thorough and systematic study of 

 this subject. 



The spectra of numerous uranyl salts, many of which were espe- 

 cially prepared for this purpose, have now been mapped. Owing to 

 the extraordinarily complex character of the phenomena, no satis- 

 factory theory has as yet been evolved, but the mass of facts here 

 recorded and the general principles established will, it is hoped, afford 

 a basis for the successful theoretical development of this important 

 and little understood branch of the science of radiation. 



PHYSICAL LABOEATORY OF CORNELL UNIVERSITY, 



May 24, 1919. 



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