150 ANNUAL OF SCIENTIFIC DISCOVERY, 



face a strong solution of nitrate of silver ; the brown image deepens 

 in intensity until it becomes black. Another change commences ; the 

 image begins to grow lighter, and, by perfectly natural magic, finishes 

 by converting the black into white, presenting the curious phenomena 

 of the conversion of a Talbotype negative into, apparently, a Daguerre- 

 otype positive, but by a very opposite agency, no mercury being pres- 

 ent ; metallic silver here (probably) producing the lights, while in 

 the Daguerreotype it produces the shades, of the picture. Prof. 

 Wheatstone has suggested the desirableness of substituting blackened 

 wood, or blackened ivory, for glass plates; we should then probably 

 have the novelty of a Daguerreotype on wood free from some of the 

 disadvantages attendant on polished metal. The application of it to 

 wood blocks, for wood engravers for certain purposes, has been also 

 suggested. 



IMPROVEMENTS IN PHOTOGRAPHY. 



THE superiority of the Talbotype to the Daguerreotype is well 

 known. In the latter the pictures are reversed and incapable of being 

 multiplied, while in the Talbotype there is no reversion, and a si igle 

 negative will supply a thousand copies, so that books may be now il- 

 lustrated with pictures drawn by the sun. The difficulty of procuring 

 good paper for the negative is so great, that a better material has been 

 eagerly sought for ; and M. Niepce has successfully substituted for 

 paper a film of albumen, or the white of an egg, spread upon glass.* 

 This new process has been brought to great perfection, and the pic- 

 tures taken by it are regarded as the finest ever executed. Another 

 process, in which gelatine is substituted for albumen, has been invented, 

 and successfully practised by M. Poitevin, a French officer of engi- 

 neers, and by an ingenious method, which has been minutely described 

 in the proceedings of the Institute of France. M. Edmond Becquerel 

 has succeeded in transferring to a Daguerreotype plate the prismatic 

 spectrum, with all its brilliant colors, and also, in an inferior degree, 

 the colors of a landscape. These colors are. however, very fugitive ; 

 yet, though no method of fixing them has been discovered, we cannot 

 doubt that the difficulty will be surmounted, and that we shall yet see 

 all the colors of the natural world transferred by their own rays to 

 surfaces of both silver and paper. But the most important fact in 

 photography is the singular acceleration of the process discovered by 

 M. Niepce, which enables him to take the picture of a landscape, illu- 

 mined by diffused light, in a single second, or at most two seconds. 

 By this process he obtained a picture of the sun on albumen so instan- 

 taneously, as to confirm the remarkable discovery, previously made by 

 Arago, by means of a silver plate, that the rays that proceed from the 

 central parts of the sun's disk have a higher photogenic action than 

 those which proceed from its margin. This interesting discovery by 

 Arago is one of a series on photometry, which that distinguished phi- 

 losopher is now occupied in publishing. Threatened with a calamity 



* See Annual of Scientific Discovery, 1S,"0, p. 112. 



