300 THE APPLICATION OF PHYSICAL FORCES. [BOOK in. 



Like Talbot's process, this method embraces two principal opera- 

 tions : first, by the aid of the camera, a negative proof of the image is 

 obtained, that is to say, an inverse image ; the lights being represented 

 by shadows, the shadows by lights, and all the half tints by mixtures 

 in the exact proportion of the extreme tints. By help of this negative 

 are taken positive proofs, in which the image resumes its normal 

 appearance ; and these proofs can be afterwards obtained in an inde- 

 finite number. 



The negative is obtained on sensitized paper, and it was chiefly in the 

 preparation of this paper that M. Blancquard-Evrard made improve- 

 ments. Instead of only covering the surface with the sensitized 

 coating, he impregnated the whole thickness with iodide of silver ; 

 placing the still moist sheet between two glasses, he exposed it at 

 the focus of the lens. The paper was obtained in the following 

 manner : 



Some very compact, thin, even and well-made paper was chosen, and 

 placed with one of its surfaces on a solution of nitrate of silver, 

 taking care that the other surface should not be moistened with the 

 liquid, and that the contact should be complete without the inter- 

 position of air bubbles. After some minutes the sheet was stretched 

 on a glass, the damp side uppermost, and left to dry in the dark. 

 The dry paper was then immersed in a solution of iodide and bromide 

 of potassium, when, a double chemical decomposition taking place, 

 two impressible substances, iodide of silver and bromide of silver, 

 were simultaneously formed, and penetrated the whole thickness of 

 the paper. 



By employing the photogenic paper whilst still moist the image is 

 impressed rapidly (an indispensable requisite in reproducing animate 

 objects, especially portraits). The dry paper requires a longer 

 exposure to the light: it is useful when travelling, for securing views, 

 landscapes, monuments, and so forth. 



The paper when taken from the camera showed a blank surface 

 like the daguerreotype plates. But here the developer is a solution 

 of gallic or pyrogallic acid, in which the sheet of paper is plunge:! 

 This organic acid reduces the iodide of silver wherever the light 

 has made an impression, and the parts thus impressed are covered 

 with a dark tint of metalic silver, distinct in proportion to the action 

 of the light. The proof is therefore negative. To render it unchange- 



