Mechanics of Luminosity. 395 



— 273° C. and 1000° as somewhat less than between 0° and 

 1000° C. (see above) at 0*037, then 



W=1273x 0037 = 47-1. 



For L we have found two values for platinum, L = 2 '3 x 10~ 4 

 and 23, whence we obtain the two values 



^=4-88xl0- 6 , ^ = 4-88xl0- 1 . 



The luminous energy forms here also in the first case only 

 a small fraction of the total energy, in the second case one 

 much larger. It is indeed not impossible that in solid bodies 

 the motions of translation of the centre of gravity, which 

 here consist in vibrations about the position of equilibrium 

 of the molecule, give rise to the production of light-vibrations. 

 That the centre of gravity cannot be greatly displaced from 

 its mean position of equilibrium follows from the occurrence 

 of interference-bands with large differences of path with solid 

 bodies *. 



Store of Luminous Energy in Photoluminescent Substances. 



41. We have already, § 37(g), indicated the possibility of a 

 more exact determination of the quantity b in certain cases, 

 e.g. in uranium nitrate. Such a determ - nation is in fact 

 possible with photophosphorescent substances. To take in 

 this case as the basis of our calculation about the same inten- 

 sity as initial intensity, which the phosphorescent substance 

 possesses at any time, might appear scarcely reasonable even 

 if the conditions of excitation were exactly determined, since it 

 depends upon the construction of the particular phosphoro- 

 scope used, the size of the openings, and of the dark space 

 between, &c. 



We must rather determine the brightness which the body 

 in question has upon constant illumination, corresponding to 

 the brightness which the light commonly called fluorescent 

 light shows. This brightness is solely dependent upon the 

 property possessed by the body of utilizing the vibrations of 

 incident light for the production of luminous energy, and the 

 rapidity with which it dies away. 



W^e have seen that the brightness of the light upon con- 

 stant illumination possesses the value 



. AJ 



V s 



b 



E. Wiedemann, Verh. der Phys.-med. Soc, in JSrlangen, July 1887. 



