164 MATHEMATICS AND NATURAL PHILOSOPHY. 



grand discovery of Daguerre "would be only a particular case. Still extending his 

 researches, M. Moser was led to a still more general inference, and the following 

 may be taken as the type of the experiments leading thereto. A plate of agate on 

 which had been engraved various figures, was placed at a distance of half a milli- 

 metre, in darkness, over a silver plate. After a few seconds, the plate was exposed 

 to mercurial vapor, and a distinct image of the figures was the result. Hence he 

 inferred the following proposition : " Any two bodies placed sufficiently near each 

 other mutually impress one another with their own image." For the physical ex- 

 planation of these phenomena, M. Moser was led to assert that all bodies are self- 

 luminous, and that they contain latent light correspondingly to latent heat. He seems 

 in this to have been seduced by a tempting but imperfect analogy, and his views 

 have not met with acceptance. Still more unfortunate. Dr. Draper imagined that 

 the results were pi'oduced by a new quality of light, to which he gave the name of 

 Tithonicity, but further researches by Snorr and R. Hunt, seemed to show that the 

 active agent in the production of these images was heat, and under the name of 

 Thermography, a new art was thus instituted, in the prosecution of which much 

 success has been attained by various experimenters. The following strange and 

 singular experiments by M. E"iepce de St. Victor, which have just appeared, de- 

 cisively demonstrate that the conclusion above accepted by the scientific world is 

 far too limited, and that bodies possess the power (somewhat allied to the known 

 phenomena of phosporescence) of giving out light absorbed by them, and even of 

 retaining this power for indefinite periods. The memoir was communicated to the 

 Academic des Sciences, by M. Chevreul, and the followiog extracts are taken from 

 a translation in the current number of the London Photographic Journal : 



Will a body which has been subjected to the influence of light or insolation, 

 preserve in darkness any efi'ects (impression) of this light ? 



Such is the problem that M. Kiepce attempted to resolve by means of photo- 

 graphy. The phosphorescence and fluorescence of bodies are known, but the 

 following experiments have never been made until now. An engraving which had 

 been kept for several days in darkness was exposed during a quarter of an hour 

 to the action of direct solar rays, one half being covered with an opaque screen. 

 The eagraving was then laid upon a sheet of very sensitive photographic paper, 

 and put in a dark place for twenty-four hours, and on beiog examined, it was 

 found that the white portions of the engraving which had not been protected by 

 the screen during its exposure to the sun, had been reproduced in black. "When 

 the engraving was kept in profound darkness for several days, and then applied to 

 the paper without being previously exposed to the sun, no result was produced. 



Certain engravings after being exposed to the action of light reproduce them- 

 selves better than others, according to the nature of the paper ; but all papers, 

 even Swedish filtering-paper with or without water-mark, reproduce themselves 

 more or less after a preliminary exposure to the light. Wood, ivory, gold-beater's 

 skin, parchment, and even the living skin, are also perfectly reproduced under the 

 same circumstances ; but not so metals, glass, and enamels. In exposing an en- 

 graving to the solar rays for a very long time, it becomes, if one may use the 

 term, saturated with light. In this way it produces the maximum of effect, pro- 

 vided that it is suffered to remain in contact with the sensitive paper, in darkness, 

 for two or three days. 



If a sheet of glass be interposed between the engraving and the prepared paper, 



