134 ANNUAL OF SCIENTIFIC DISCOVERY. 



picture being developed by pyro-gallic acid. The extraordinary sen- 

 sibility of this preparation may be inferred from the fact, that a posi- 

 tive copy, from a glass negative, has been obtained in five seconds by 

 gas-light. The film formed on glass is far more adherent than the ordi- 

 nary collodion or albumen. 



Hyalotype. A new process of taking photographs upon glass has 

 been invented by Messrs. Langenheim, of Philadelphia. The most 

 interesting application of this discovery is stated to be the formation 

 of pictures upon magic-lantern slides, taken from nature by the camera 

 obscura, without the aid of pencil or brush. These pictures are pre- 

 pared by the action of light alone ; and the smallest details are deline- 

 ated on the glass with astonishing fidelity. When these slides are 

 magnified in the magic lantern, they give a perfect representation of 

 nature, and are perfectly free from all those defects and inaccuracies 

 existing in the old slides, which can never be avoided in painting upon 

 so small a scale. 



IMPROVEMENTS IN CAMERAS FOR DAGUERREOTYPES. 



M. EVRARD, of Paris, in experimenting on the proper form of the 

 camera, as used for taking daguerreotypes, has ascertained, contrary to 

 what has been the opinion heretofore on the subject, that the black 

 coating inside of all the cameras now used, to prevent the reflection of 

 light, lessens the photogenic action on the prepared plate or paper. 

 He has, therefore, lined the sides of his camera with white paper, and 

 given the interior of his tube a white coat, at the extremities of which 

 are the two object-glasses. With the above alterations in his instru- 

 ment, he has experimented on the silver plate, albumenized glass, and 

 on paper, and he states that the image forms in one half the time 

 required in the blackened camera ; that it is formed by light insuf- 

 ficient to give an image in the usual camera ; that the action is more 

 uniform the light parts not disappearing before the shaded parts are 

 fully impressed and there is far less resistance to photogenic action 

 in red, yellow, and green colors. 



Sir David Brewster, in noticing in a late publication this discovery 

 of Evrard, remarks : k ' It is not easy to explain the results obtained 

 by M. Evrard ; the effect of internal light on the negative must be to 

 darken the whole of the negative paper, and, consequently, to accele- 

 rate the production of all the lines which constitute the picture ; but 

 if the light acts equally upon the dark lines when they are darkening, 

 as it does upon the light parts, the depth of color of the black lines 

 cannot be increased, because the depth of the ground on which they 

 are drawn is equally increased. The internal light must, therefore, 

 darken the dark parts of the negative more than the light parts. It is 

 obvious that the internal light scattered over the negative cannot be 

 uniform. It would, therefore, be better to keep the camera black, as 

 hitherto, and to admit light through one or more apertures, so as to 

 illuminate equally the surface of the negative. This might be done 

 either through ground glass or paper, or by reflection from any white 



