PHOSPHORESCENCE SPECTRA. 55 



The slit of the latter instrument was divided into two parts by means 

 of an opaque strip across the middle (S, fig. 43) . 



Within the collimator a doubly refracting rhomb R and Nicol 

 prism N were mounted. The rhomb gave two slit-images vertically 

 displaced and the adjustment was such that the lower part (A) of one 

 image was contiguous with the upper part (B) of the other. 



Thus two spectra of the phosphorescent field were obtained corre- 

 sponding to the two polarized components. These presented the usual 

 distinctive structures at whatever stage of the phosphorescent decay 

 they were observed. By rotation of the Nicol prism the two fields 

 could be brought to equal brightness for any given part of the spec- 

 trum, and this balance, if made with the sector of the phosphoroscope 

 set so as to give observations at 0.0005 second after extinction, was 

 found equally correct up to 0.005 second or as long as phosphorescence 

 was observable. The two components therefore decay at the same 

 rate. 



SUMMARY OF PHOSPHORESCENCE OF SHORT DURATION. 



(1) All uranyl salts thus far examined possess the same type of 

 phosphorescence; i. e., with increasing instead of diminishing rates of 

 decay. 



(2) This is true not only of the crystalline forms, but also of uranyl 

 compounds in solid solution or in the plastic state characteristic of the 

 double phosphates. 



(3) The initial brightness of phosphorescence under like excitation 

 varies greatly with the different salts, as does also to some extent the 

 rate of decay. 



(4) The brightness of a salt newly prepared in darkness is greater 

 when first excited than subsequently, but it soon reaches a nearly 

 stable condition. 



(5) Exposure to red and infra-red rays is without effect as regards 

 the rate of decay. 



(6) The phosphorescence, like the fluorescence, of the uranyl salts 

 appears to be independent of the mode of excitation, and the structure 

 of the intricate spectrum is the same during excitation and throughout 

 the observable phosphorescent interval. 



(7) Changes in the rate of decay are not continuous, but occur in 

 definite steps, there being at least three successive processes within the 

 interval covered by observations, i. e., about 0.006 second. These 

 processes follow a law such that 7-1/2 i s j n linear relation to the time. 



(8) The first and second processes, counting from the close of 

 excitation, are of nearly equal duration, increasing in duration with 

 the intensity of excitation in such a manner that the duration of the 

 process is approximately proportional to the natural logarithm of the 

 excitation. 



