PHYSICS — L13WIS, NICHOLS. 21/ 



violet spectra of compounds. This work is now temporarily suspended in 

 order to make such chano;es in the apparatus that it may be used in the 

 Crocker eclipse expedition of the Lick Observatory, in which the writer has 

 been invited to take part. 



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

 286. Quantitative study of fluorescence and phosphorescence, especially 

 at low temperatures. (For previous reports see Year Book No. 4, p. 

 254, and Year Book No. 5, p. 228.) $1,000. 



The work of the past year under this grant falls under three heads as 

 follows : 



( 1 ) Effect of red and infra-red rays upon photo-luminescence. Observa- 

 tions have been made upon two different specimens of Sidot blende. Results 

 have been obtained concerning — 



(a) The effect of the longer rays before the excitation of the screen, i. e., 

 the effect of these rays in destroying the semi-permanent change in Sidot 

 blende that is left behind after the excitation and decay of phosphorescence. 

 This change is evidenced by the fact that the effect of a given excitation will 

 ordinarily be influenced by the intensity and duration of a previous excitation, 

 even after the lapse of several days. Exposure to certain infra-red rays 

 restores the Sidot blende to a standard condition. We have investigated the 

 influence upon this effect of the intensity and duration of exposure to infra- 

 red. 



(b) The effect of the longer waves during excitation, i. e., the effect 

 upon the fluorescence of Sidot blende. The effect has been measured by 

 means of a spectrophotometer throughout the luminescence spectrum. 



(c) The effect of the longer waves after excitation, i. e., during the decay 

 of phosphorescence. The modified form of the decay curve has been deter- 

 mined in num.erous cases and under various conditions. The character and 

 relative magnitude of the effect in different parts of the luminescence spec- 

 trum were also determined. 



{d) The relative effect of dift'erent wave-lengths in the red and infra-red 

 spectrum in accelerating the decay of phosphorescence. 



The work falling under the above heads is ready for publication and will 

 appear in the October or November Physical Review (1907). 



(2) Fluorescence absorption. The phenomenon known as fluorescence 

 absorption, i. e., the increase in the absorbing power of a substance during 

 phosphorescence, was discovered by Burke in the case of uranium glass, and 

 has been studied in greater detail by us in the case of several other 

 fluorescent substances. All of the results on this subject have been called in 

 question by Carmichel, who fails to detect the phenomenon. We have there- 

 fore repeated our experiments with a new spectrophotometer, and by an 

 entirely different method. Our earlier results were entirely confirmed. A 



