PREFACE. 



The series of investigations described in this memoir was begun in 1903. 



The authors believed that by the use of quantitative methods and par- 

 ticularly by the application of the spectrophotometer to the study of the 

 spectra of fluorescent and phosphorescent substances, something of definite 

 value might be added to the existing information concerning luminescence. 



Spectroscopy, whether visual or photographic, is a method of high 

 precision where applied to line spectra, but in the case of the broad bands 

 of the spectra of fluorescent solids and liquids it affords but little informa- 

 tion beyond the approximate width and general location of the bands. The 

 spectrophotometer, on the other hand, enables the observer to determine 

 the distribution of intensities throughout the emission bands and the coeffi- 

 cients of absorption for the various wave-lengths of the corresponding 

 absorption bands. From the curves expressing the results of such measure- 

 ments, moreover, it is possible to locate with considerable accuracy the 

 crests of the bands. One may thus attain some detailed knowledge of the 

 laws of the radiation of luminescence, compare luminescence with the radi- 

 ation due to temperature, and obtain a basis for theoretical discussion. 



A number of important portions of the work described in this volume have 

 been carried out at our suggestion by Doctors Frances G. Wick, C. A. 

 Pierce, Percy Hodge, and C. W. Waggoner, and by Messrs. H. E. Howe 

 and Carl Zeller. To these investigators and also to Prof. W. R. Orndorff, 

 who has repeatedly aided us by undertaking the preparation of fluorescent 

 compounds and by suggestions concerning the chemical aspects of the 

 problem, we desire to express our indebtedness. 



The recent exhaustive, thorough, and discriminating review of the very 

 large literature relating to luminescence published by Professor Kayser 

 (H. Kayser, Handbuch der Spektroskopie, Bd. iv, Kap. v and vi) in his 

 Handbook of Spectroscopy, makes any extended bibliographic or historic 

 treatment here unnecessary and we have therefore given only such refer- 

 ences to previous researches as bear directly upon the subjects under 

 consideration. 



The subject-matter contained in the several chapters appeared from 

 time to time, as each portion of the work reached completion, in a series 

 of papers in the Physical Review. In gathering this material together in a 

 single treatise we have recast and rearranged it, but have preserved the 

 original form of presentation in so far as it was found to be consistent with 

 our views after the completion of the work. 



Grants which were received from the Carnegie Institution of Washington 

 in 1905, 1909, and 1 9 10 have greatly facilitated the prosecution of the experi- 

 ments described in this memoir and of others now in progress and have 

 furthered the preparation for investigations which it is proposed to under- 

 take in the near future. 



Physics Laboratory of Cornell University, 



May 23, iqii. 



in 



