DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE- 57 



54 and 55 shows that the differences between the computed and observed 

 values of /, which are usually small, are entirely unsystematic. It is 

 possible to test the law still further by considering the values of the con- 

 stants a and b. Except in the case of the last curve of Table 13, the data 

 refer to curves of decay for different wave-lengths, but with the same 

 excitation. a and are therefore constant, while k depends upon the 

 wave-length. It will be noticed that 



a 1 II 1 



This quotient should therefore be a constant for all curves taken with the 

 same excitation and under similar experimental conditions. The values 

 of the ratio a/b for the three curves of Table 15 show considerable vari- 

 ation from equality, being 1.5, 2.0, and 1.7. The values of the ratio for the 

 first three curves of Table 13 are 1.73, 1.75, and 1.65. In view of the 

 difficulty of maintaining constant conditions in the experiments on phos- 

 phorescence the agreement between these three values is highly satisfactory. 

 As already stated, the observations at 0.483 ju in Table 14, probably on 

 account of some experimental error, can not be represented by the expres- 

 sion here considered, so that the value of a/b for this curve can not be deter- 

 mined. But for the curves taken at 0.512 n and 0.547 M (Table 14) the 

 quotient a/b has the values 2.32 and 2.43, respectively. Here, too, the 

 agreement is all that could be expected. 1 Our results therefore afford 

 strong confirmation not only of the conclusion that all parts of the green 

 band decay at the same rate, but also of the general theory of phosphor- 

 escence that we have used in deriving the law of decay. 



The data permit still another test of the general theory, the result of 

 which is less satisfactory. It will be remembered that in the case of the 

 fourth curve of Table 13 the exciting light was less intense than that used 

 with the other curve for 0.512 (jl. The observations for the fourth curve 

 were made on the same day as those for the second curve, and although the 

 former were made in the afternoon and the latter in the forenoon, every 

 attempt was made to keep the comparison source the same in the two cases 

 and to have all the other experimental conditions as nearly as possible 

 constant. If we compare the two curves for 0.5 1 2 y, in Table 13, we see 

 that a and k have the same value for both, while n is different. It would 

 seem reasonable, however, to regard the constant b as independent of 11 0. 

 This constant should therefore have the same value for both curves; in other 

 words, the straight lines obtained by plotting 7~- should be parallel. As a 

 matter of fact the values of b determined for the two curves are 0.045, for 

 the second curve of Table 13, and 0.052 for the fourth. The fact that the 

 two values are unequal is shown in Fig. 45 by the lack of parallelism of the 

 two lines A' and B'. 



Both the theory of Becquerel and the simple theory first discussed in 

 Chapter XV fail to explain this result; which, however, is in accord with the 

 modified theory of Wiedemann and Schmidt discussed in the latter part of 

 Chapter XV. 



'There is no reason why the value a/b for Table 13 should be the same as those for Table 14 or 15, since 

 the observations were made several weeks apart, and with no attempt to keep the intensity of the com- 

 parison source the same in the different cases. 



