PHYSICS — NICHOLS. 229 



whole, have been made by E. Becquerel and others, and their results show a 

 more complicated relation than that which we have found. 



Since the law of decay for various wave-lengths of the phosphorescence 

 spectrum is the same, it was possible to apply our method with some modifi- 

 cations to the undispersed light, and thus to greatly increase the interval of 

 time under observation. 



By means of a specially devised photometer used in connection with a 

 chronograph we have made an extended study of the decadence of the phos- 

 phorescence from calcium sulphide, Sidot blende, willemite, and other phos- 

 phorescent materials under a variety of conditions. The interest and novelty 

 of the results thus obtained was such as to lead us to spend a great portion 

 of the year on this phase of the subject and to plot hundreds of decadence 

 curves. The decadence of phosphorescence is in fact a somewhat compli- 

 cated phenomenon, depending upon the length, intensity, and character of 

 excitation and Hkewise upon the previous history of the substance. The 

 emission of light after excitation appears to result from two independent proc- 

 esses, one of which succeeds the other after a fixed interval of time. The law 

 of the two processes is the same, being that determined in our earlier experi- 

 ments, but the constants of the equation for the relation between time and 

 intensity of light are different. The effect of increasing the intensity of excita- 

 tion on the one hand or of prolonging the duration of a weak excitation upon 

 the other is to produce saturation somewhat analogous to that observed in 

 the magnetization of iron and even an effect which might be termed hyste- 

 resis. The results of these further studies of the decay of phosphorescence 

 have been given in a paper printed in the Physical Review* for July, 1906. 



The remarkable effect of certain of the longer wave-lengths of the spec- 

 trum, particularly of the red and infra-red regions, upon phosphorescence 

 is likewise under investigation. A mass of interesting results has been 

 obtained, and the experiments, which were interrupted for a time by the 

 removal of the Department of Physics to its new laboratory, are again under 

 way. The preliminary report upon this portion of our subject was made at 

 the July meeting of the Physical Society. 



Apparatus for a thorough investigation of fluorescence and phosphor- 

 escence under the action of the cathode rays has been purchased from money 

 made available by this grant, and the experimental work will be taken up as 

 soon as the study of the effect of the infra-red is completed. 



Under the direction of Professors Nichols and Merritt a graduate student, 

 Miss Frances G. Wick, has made a careful spectrophotometric examination 

 of the optical properties of a typical fluorescent solution and a much more 

 thorough and systematic study of the phenomenon of fluorescence absorp- 

 tion than had hitherto been attempted. This phenomenon, the existence of 



*Physical Review, vol. 23, p. 37. 



