NATURAL PHILOSOPHY. 139 



engraving had been applied direct!}' upon the sensitive paper, but yet 

 distinct. 



When a tablet of black marble, lightly stre\vn with white spots, after 

 having been exposed to the light, is applied at once to a sensitive paper, the 

 white parts of the marble only are imprinted upon the paper. Under the 

 same conditions, a tablet of white chalk will produce a sensible impression, 

 while a tablet of charcoal Avill produce no such effect. When a black and 

 white feather has been exposed to the sun, and applied in darkness to a 

 sensitive surface, the white parts alone imprint their image. The feather 

 of a parrot red, green, blue, and black has gi\'en scarcely any impres- 

 sion, acting as if the feather had been black. Certain colors, however, have 

 left traces of a very feeble action. 



Experiments have been made with textile fabrics of different natures and 

 of various colors. The following are a feAV of the results : 



Cotton White impressed the sensitive paper. 



" Brown (by madder and alumina). Nothing given. 



Violet (by madder, alumina, and iron). Scarcely anything. 



Red (by cochineal). Nothing. 



Turkey Red (by madder and alum). Nothing. 



Prussian Blue, upon white ground, is the blue which produces 

 the best impression. 



Blue (by indigo). Nothing. 



Chamois (by peroxide of iron). No impression. 



Linen, silk, and woollen cloths giA'e equally different impressions, accord- 

 ing to the chemical nature of the colors. 



M. Nie'pce calls particular attention to the following experiment, which is, 

 as he says, curious and important : 



We take a tube of metal of tin-plate, for example, or of any other 

 opaque substance closed at one of its extremities, and cover the interior 

 with paper or AA'hite card ; the open end of the tube is exposed for about an 

 hour to the direct rays of the sun. Then apply this open end to a sheet of 

 sensitive paper, and preserA r e it in this state for twenty-four hours, when the 

 circumference of the tube will have designed its image. More than this. If 

 an engraving upon china paper is interposed between the tube and the sensitive 

 paper, we Jind the same reproduced. Reproduced, be it remembered, by the 

 radiations which have been absorbed and redeveloped from the interior of 

 the tube. " If we close the tube hermetically as soon as AVC cease to .expose 

 it to the light, we shall preserve, during an indefinite time, the faculty of 

 radiation, which the insulation has communicated, and we shall see that this 

 is manifested by the impression produced Avhen we apply the tube upon a 

 sensitive paper, after having removed the cover by which the tube AA'as 

 closed." 



Xiepce then informs us that he has repeated upon images formed in the 

 camera-obscura similar experiments to those Avhich he has made with the 

 direct light. A piece of card which had been kept in darkness was placed 

 in the camera-obscura for about three hours, and on it was projected an 

 image brilliantly illuminated by the sun. Then the card was applied to sensitive 

 jjaper, and after twenty-four hours there was obtained a reproduction of the 

 primitive image of the camera-obscura. There must be a long exposure to 

 obtain an appreciable result. 



It will be remembered that, some few years since, Professor Stokes drew 



