74 INVISIBLE 



paper. I have seen tubes of this kind that had 

 been prepared and corked up for a week, a fort- 

 night, and some that had even been closed up for 

 months, after their exposure to light, and all left 

 the impression of their orifices upon the photo- 

 graphic paper, as if the paper in the tube had 

 been acted upon only a few seconds before. But 

 the impression is not so intense when the tube 

 has been kept closed for a long period of time. 



These effects are owing probably to a pheno- 

 menon of phosphorescence, and, if so, they prove 

 evidently that all bodies possess this property to 

 a greater or less extent, depending upon the 

 nature of the substance examined. Luminous 

 vibrations, which constitute phosphorescence, are 

 hereby shown to exist when we cannot perceive 

 them : their presence is made known by the pho- 

 tographic paper when the eye is not able to discern 

 them. These luminous vibrations persist, also, for 

 a period of time which is much longer than any 

 one would, at first, be inclined to suppose. 



In a recent paper, M. Niepce says : " I have 

 repeated my former experiments of shutting up 

 light in tubes, employing in these experiments 

 cardboard imbibed with a salt of uranium, or with 

 tartaric acid. The results have been far more 

 surprising than before. I expose to sunlight a 

 sheet of cardboard saturated with tartaric acid or 

 with a salt of uranium, after which I roll my card- 



