NATURAL PHILOSOPHY. 137 







was suggested by Mr. Grove. As telescopic power is known to be limited by 

 the area of the speculum or object-glass, even assuming perfect definition, as 

 the light decreases inversely as the square of the magnifying power, a limit 

 must be reached at which the minute details of an object become lost for want 

 of light. Now, assuming a high degree of perfection in astronomic photo- 

 graphs, these may be illuminated to an indefinite degree of brilliancy by ad- 

 ventitious light. With a given telescope, could a better effect be obtained, by 

 illuminating the photographic image, and applying microscopic power to 

 that, than by magnifying the luminous image in the usual way by the eye- 

 glass of the telescope ? Can the addition of extraneous light to the photo- 

 graph permit a higher magnifying power to be used with effect than that 

 which can be used to look at the image which makes the photographic im- 

 pression ? In other words, is the photographic eye more sensitive than the 

 living eye; or can a photographic recipient be found which will register im- 

 pressions which the living eye does not detect, but which, by increased light 

 or by developing agents, may be rendered visible to the living eye? The 

 phenomena treated of, which are a mere selection from a crowd of analogous 

 effects, shoAV that light and electricity, in numerous cases, produce a molec- 

 ular change in ponderable matter affected by them. The modifications of 

 the supposed imponderables themselves have long been the subjects of inves- 

 tigation ; the recent progress of science teaches us to look for the reciprocal 

 effects on the matter affected by them. Few, indeed, if any, electrical effects 

 have not been proved to be accompanied with molecular changes ; and AVC 

 are daily receiving additions to those produced by light. Mr. Grove feels 

 deeply convinced that a dynamic theory, one which regards the imponder- 

 ables as forces acting upon ordinary matters in different states of density, 

 and not as fluids or entities, is the truest conception which the mind can 

 form of these agents ; but to those who are not willing to go so far, the 

 ever-increasing number of instances of such molecular changes affords a 

 boundless field of promise for future investigation, for new physical discov- 

 eries, and new practical applications. 



NIEPCE DE ST. VICTOR'S DISCOVERIES IN REGARD TO THE ACTION 



OF LIGHT. 



The following is a resume of the highly important discoveries recently 

 made by M. Xiepce de St. Victor, in regard to the action of light, as com- 

 municated by M. Chevreul to the French Academy. 



The conditions now determined are that any body, after having been ex- 

 posed to light, retains in darkness some impression of this light. M. Nie'pce 

 remarks "The phosphorescence and the fluorescence of bodies are well 

 known, but I am not aware that any experiments have ever been made on 

 the subject which I am about to describe." 



Expose to the direct rays of the sun, during a quarter of an hour at least, 

 an engraving which has been kept many days in obscurity, and of which 

 one-half has been covered by an opaque screen; then apply this engraving 

 upon a very sensitive photograph paper, and, after twenty-four hours' contact 

 in darkness, we shall obtain, in black, a reproduction of the white parts of 

 the engraving, which, in the process of insulation, has not been sheltered by 

 the screen. 



If the engraving has been kept for many days in profound darkness, and 

 we then apply it upon sensitive paper, without having previously exposed it 



12* 



