M. C. Lea on a Theory of Photo-chemistry. 75 
ent phosphorescent bodies emit light of different colors. If one 
os agree body emits a red, another a green, another a 
trum. In fact this is so completely within the analogy of the 
posi- 
tion in presence of light, then, so long as this phosphorescence 
i inescence conti 
: , this acta | posi 
tion will take place. If simply left in darkness, this actines- 
cence will, as I have already shown, gradually expend itself, 
Precisely like ordinary phosphorescence. And also as in the 
case of ordinary phosphorescence, a fresh exposure to light will 
create a fresh impression, the iodid of silver having apparently 
ond exposure be made under a photographie negative, and an 
Sead pataalg developer is applied, a clear sharp image 
obtain | 
It seems worthy of remark that though no visible phospho- 
rescence is noticeable in the case of iodid of silver, it is by no 
Means impossible that if examined by the ingenious instrum 
constructed for Mr. Becquerel by Mr. Dubose, a visible phos 
Phorescence might also be observed. However this may be, it 
does not affect the principles here laid down, 
