158 Prof. E. Wiedemann on the 



a part of which is lost by external radiation on the free path 

 between two impacts. 



The two cases require separate consideration, for the mode 

 of excitation is essentially different. In incandescence it is 

 the reciprocal relationships between the impinging molecules, 

 w r hether of the same, or of different kinds, which produce the 

 motions causing light ; while in photo- and electro-lumines- 

 cence these are due to an external motion affecting the 

 particles. 



(b) The loss in luminous energy may also have various causes. 

 A loss of energy ensues in consequence of the issuing light- 

 vibrations ; further, in luminescent bodies in consequence of 

 the impact of two molecules a part of the energy of the lumi- 

 nous motions may be converted into energy of translation and 

 thus produce a rise of temperature. Further, within each 

 molecule only those atoms associated in a particular way, the 

 chromogenic, which we will here call lucigenic, may perform 

 luminous vibrations together. These motions may in part be 

 transferred to the neighbouring non-lucigenic atoms, and may 

 thus suffer diminution. According to the structure of the 

 molecules, only a particular kind of vibrations will be checked 

 in any high degree, whilst others will remain undisturbed— a 

 process to which numerous analogies are well known in 

 acoustics. 



We have appropriate examples amongst the phenomena of 

 light in the observations on solutions of fluorescein and 

 eosin in gelatine, which I have previously communicated*, 

 where the gelatine is mixed with solutions of the substances 

 and allowed to dry. In the fluorescent light yielded by these 

 substances, observed while illuminated, the spectrum appears 

 almost continuous from red to green. On the other hand, the 

 phosphorescent light observed some time after the illumination 

 has ceased, shows a very dark band in the orange. The phos- 

 phorescent light was examined in the phosphoroscope de- 

 scribed by me, the arrangement being such that the phospho- 

 rescent light was examined from the same side as that on 

 which the incident light fellf. The absorption of the light 

 excited before the observation was diminished as much as 

 possible. We must conclude from these observations that in 

 these bodies the loss of luminous energy for the complex of 

 rays in the orange is determined, not only by radiation, but 

 also by an absorption within the molecule itself. 



13. We will now consider the intensity-relationships of the 

 light emitted by a body, and will investigate the two cases : 



* Sitzungsber. d. physikal-med. Soc. Erlangen, July 1887. 

 t Wied. Ann. xxxiv. p. 453 (1888). 



