CHAP, vii.] PHOTOGRAPHY ON PAPER AND ON GLASS. 299 



we have thus obtained calotype paper. It is placed at the focus of the 

 camera, one minute suffices to imprint the image, which appears with 

 all its details, when, after having washed the paper in the gallonitrate of 

 silver, we warm it gently before the fire. To fix the picture it must 

 be moistened with a solution of bromide of potassium, and again washed 

 and dried. Drawings fixed in this way remain transparent, and they 

 may be copied on another sheet of calotype paper, which is pressed 

 against the picture and thus exposed to the light." 



In this process we find the same physical principles as in those 

 of Niepce and Daguerre. A sheet of paper is covered with a sensitized 

 coating impressible to the light; it is submitted to this influence at 

 the focus of a camera. Still invisible when removed from the camera, 

 the image requires the action of a special substance, of an operation 

 which will develop x it ; finally, to preserve it from causes of ulterior 

 destruction, a last operation is necessary, that of fixing. 



All the subsequent photographic processes, and they are number- 

 less, are based on the same principles and necessitate the same 

 fundamental operations. 



In what particulars, then, did Talbot, who at first furnished proofs 

 in many points defective, show an advance in the new art ? In this. 

 Daguerre's plates were heavy and expensive, embarrassing when travel- 

 ing, and awkward in manipulation. Besides, the image, notwithstand- 

 ing its admirable accuracy and the finish of its details, has a dazzling 

 reflection, which makes it difficult to examine ; one can only see it 

 under certain conditions of light. Moreover, one proof is the only result 

 of the operation, which must be recommenced as often as fresh copies 

 of the same object are required. On all these points, but especially 

 on the latter, that of reproducing copies from the same proof, the 

 process of Talbot showed a considerable progress, and this progress 

 was practically realized in a few years. 



First, M. Blancquard-Evrard, of Lille, succeeded, by improving 

 on Talbot's process, in obtaining more and more perfect proofs on 

 paper, and while improving the results, he found means of succeeding 

 almost without failure, a thing which could not be said for the process 

 described above. Let us succinctly describe his method. 



1 The development of images by gallic acid and nitrate of silver was the dis- 

 covery of the Kev. J. B. Reade, from whom we may suppose Fox-Talbot borrowed 

 it for the autotype process. 



