228 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Nichols, Edward L., Cornell University, Ithaca, New York. Grant No. 286. 

 Qtia7ititative shidy of fluorescence and phosphorescence^ especially at low 

 temperatures. (For previous report see Year Book No. 4, p. 254.) 



$1,000. 



Report. — Investigations have been in steady progress by Professors Nichols 

 and Ernest Merritt. The substance known as Sidot blende, a fluorescent 

 zinc oxide, which seems to be especially well suited to bring out the rela- 

 tionships that exist between different types of luminescence, has been made 

 a subject of detailed study. The luminescence of Sidot blende when excited 

 by Roentgen rays was carefully studied with the spectroscope, and the results 

 were compared with the photo-luminescence of the same substance during 

 excitation by means of the light of the carbon arc. To determine the 

 validity or failure of Stokes's law a pure spectrum from the arc light was 

 obtained by the method of double dispersion, and it was found that lumi- 

 nescence could be excited by light of wave-lengths lying between points 

 0.47 iJ. and 0.497 '") whereas the phosphorescence spectrum could be followed 

 to wave-length 0.46 /x. It appears, therefore, that there is a violation of 

 Stokes's law similar to that already noted in the case of similar fluorescent 

 substances. 



These experiments were followed by a careful study of the law of deca- 

 dence of the phosphorescence spectrum of Sidot blende, wave-length by 

 wave-length. It was found that the phosphorescence spectrum remains 

 unaltered during decay as regards the distribution of intensities, the rate of 

 decay of the various wave-lengths being the same. A preliminary account 

 of these experiments was published in the Physical Review* for October, 1905. 



This work was followed by an extended series of experiments upon the 

 law of decay of the phosphorescence of Sidot blende, a subject of consider- 

 able importance to the general theory of luminescence. Experiments were 

 first made upon various wave-lengths of the phosphorescence spectrum, 

 measurements of the brightness of each region being made by means of the 

 spectrophotometer as a function of the time which had elapsed after the close 

 of excitation. It was found that the results could be closely represented by 

 an expresssion of the form 



i/|/T'=« + bt 

 where a and b are constants. 



A preliminary account of these experiments was published in the Physical 

 Review t for May, 1906, in which paper the bearing of the relation deter- 

 mined upon theories of luminescence is discussed at considerable length. 



In these measurements it is possible to extend observations only over a 

 brief period of time — about seven seconds. Early observations of the decay 

 of phosphorescence of other substances, taking the phosphorescent light as a 



*Physical Review, vol. 21, p. 247. t Physical Review, vol. 22, p. 279. 



