348 Mr. T, A. Malone, on the Application of Light [Feb. 18^ 



also been applied to steel plates by M. Mante, in a series of natural 

 history plates, published in Paris, and also by M. Niepce de St. 

 Victor, in the frontispiece to his treatise. 



In 1854, Herr Paul Pretsch, of the Imperial printing office of 

 Vienna, patented in this country, and subsequently in France, a 

 process which he has called Photogalvanography. He uses Mr. 

 Talbot*s materials, but with certain additions, and avails himself of 

 a property of the gelatine which allows of his dispensing with the 

 acid etching altogether. We are unable to speak with certainty of 

 the exact comparative merits and capabilities of the two processes. 

 Mr. Talbot's results are on steel ; Herr Pretsch's on copper. If 

 other things be equal, the steel would possess the advantage of 

 greater durability. Herr Pretsch takes one part of clear gelatine 

 or glue, and about ten parts of water to form a jelly, which he 

 mixes with a strong solution of bichromate of potash ; to this mix- 

 ture he adds a fresh portion of jelly, containing nitrate of silver in 

 solution ; the whole being warmed and thoroughly mixed for about 

 ten minutes. He next adds a third portion of jelly, containing a 

 comparatively small quantity of iodide of potassium ; then the whole 

 mixture is strained, and is ready to coat the glass plates which are 

 at first used in this process. A plate being coated and dried, is 

 applicable to all the purposes enumerated in the early bitumen 

 process. It can be used to copy engravings by superposition, or 

 be made to receive the images of the camera. However, it is found 

 that the most practical way to make use of the bitumen and gela- 

 tine processes, is to copy from a positive photograph which has 

 resulted from a collodion or a Talbotype negative. We have only 

 to place the positive print upon the dried orange-coloured jelly, 

 press it in contact by a plate of glass, and expose the whole to the 

 light for some time, when we shall find upon removal that we have 

 obtained upon the dried jelly a photographic representation of the 

 positive print. Wherever the light has acted strongly the plate 

 will have changed from its bright orange-red colour to a more 

 tawny hue, this latter shade of colour gradually passing in the half 

 tints into the unaltered red of the parts completely shielded from 

 the light. The parts acted upon by the light have now become, 

 as in Mr. Talbot's case, comparatively insoluble in water. So 

 far, Herr Pretsch's process has much in common with Mr. Tal- 

 bot's, but the two experimenters now diverge widely. Herr 

 Pretsch, instead of dissolving away the unaltered jelly, merely 

 soaks the plate in water long enough to cause the unaltered gelatine 

 to swell, and so to rise above the surface in such a way that we 

 obtain a picture in relief resembling the condition of an ordinary 

 cut wood-block. The tawny coloured parts do not swell, and so 

 they remain depressed, representing the sunken portions of the 

 wood-block. If the swelled gelatine were hard enough, we might 

 at once ink the raised parts by a roller, and print in the usual way. 

 This, however, is impracticable ; and, moreover, surface printing is 



