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. Nicpce 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 | roll my card- 
