396 Prof. E. Wiedemann on the 



where J is the brightness of the incident light, and A a con- 

 stant dependent upon the nature of the body. 



The brightness determined in a phosphoroscope of the 

 former construction * is 



is= -^(l—e-^)- t — j-,e~ bS . 



b K J I— er hT e- bT 



t is the time during which the body is illuminated, t' the 

 time during which it is not illuminated, S the time between 

 the end of the illumination and the beginning of the observa- 

 tion, p the time during which the body is observed. 



In the apparatus which I used, 



T = d>, t'=38, t + t' = 4S, p = 8, 



then 

 where 



AJ (1 -«-")» AJ 



b (l-e~ iM ) ~ b h 



J 1 1 „-4bS\ e 



l-e~* bS ) 



represents the constant independent of the velocity of rotation. 



I will consider later on the properties of the function /, 

 which exhibits a maximum value. We see at once that with 

 decreasing velocity of rotation, that is with increasing S, the 

 function / first increases and reaches a maximum for a 

 value of b$ = l'5 about, and then diminishes, at first rapidly 

 and afterwards more slowly. The smaller b is, the greater is 

 8 when the maximum occurs. If b is very small, i, e. if the 

 decrease in intensity takes place very slowly, the brightness 

 appears scarcely altered with great changes in the velocity of 

 rotation. One would also come to the same conclusion as to 

 the existence of such a maximum of brightness by simply 

 considering the nature of the excitation of light and the 

 expenditure of light. 



In order to find b we determine the intensities i s for a 

 series of different velocities of rotation, i. e. for different 

 values of S. From the values obtained from each two 

 measurements we calculate b, and thence for a definite value 

 of S the factor/. By dividing the value of / into the corre- 

 sponding value of i$ we obtain the value of the intensity of 

 the fluorescent light F : if=AJ/b.~\ 



* E. Wiedemann, Wied. Ann. xxxiv. p. 458 (1888). 



t By observations of the phosphorescent light with different velocities 

 of rotation, and by a similar line of reasoning, it is not in general possible 

 to determine the brightness of the fluorescent light of crystals &c. Whilst 

 with liquids and the different sorts of glass it is possible, at least to a 

 very great extent, by forming them with plane surfaces, to reflect the 



