150 ANNUAL OF SCIENTIFIC DISCOVERY. 



potash, The object to be engraved, which " may be cither material sub- 

 stances, as lace, the leaves of plants, etc., engravings, writings, or photo- 

 graphs, etc.," is then placed on the prepared plate, and both are screwed in 

 a photographic printing-frame. After exposure to sun-light, or for a longer 

 period to the common daylight, as is usual in photographic printing, the 

 plate is removed from the frame, when " a faint image is seen upon it the 

 yellow color of the gelatine having turned brown wherever the light has 

 acted." Thus far, the process is precisely the same as that of Mr. Talbot's 

 invention, patented in October, 1852. But in that process the next stage was 

 to wash the plate in water, or water and alcohol, in order to dissolve that 

 portion of the gelatine on which the sun had not acted, and in so doing the 

 image has almost invariably been found to be injured. In the new method 

 the plate is not washed at all, but the operator proceeds at once and in 

 this it is that the novelty of the new method mainly consists to cover the 

 surface of the plate evenly with a little finely-powdered gum copal, which is 

 then melted by holding the plate over the flame of a spirit lamp. A uniform 

 aquatint ground is thus formed, and as soon as it is cold the plate is ready 

 for etching. If Mr. Talbot's specification be sufficient, the etching process 

 is the simplest of any yet practised. The etching liquid is a solution of 

 perchloride of iron five or six parts of the saturated solution to one of 

 water. A small quantity of this is poured upon the plate, and with a camel's 

 hair brush spread equally all over it. " The liquid penetrates the gelatine 

 wherever the light has not acted on it, but it refuses to penetrate those parts 

 upon which the light has sufficiently acted. It is upon this remarkable fact 

 that the art of photoglyphic engraving is mainly founded. In about a 

 minute the etching is seen to begin, which is known by the parts etched 

 turning dark-brown or black, and then it spreads over the whole plate the 

 details of the picture appearing with great rapidity in every part of it." If all 

 proceeds well, the details of the picture will present a satisfactory appearance 

 to the eye of the operator in two or three minutes ; the operator stirring the 

 liquid all the time with a camel's hair brush, and thus slightly rubbing the 

 surface of the gelatine, w r hich has a good effect. When it seems likely that 

 the etching will improve no further, it must be stopped, and the plate cleaned, 

 when the etching is found to be completed. 



The etching process, as Avill have been seen, is finished at once. No 

 " stopping out " even of the more delicate parts, as in ordinary engraver's 

 etching, would seem to be necessary; at least none is mentioned. In order 

 to bring out faint parts, or to deepen others, we are told, however, that the 

 operator may " touch with a camel's hair brush, dipped in liquid (Xo. 3), 

 those points of the picture where he wishes for an increased effect." The 

 No. 3 liquid is merely la weaker solution (equal parts of water and the 

 saturated solution) of the perchloride of iron for it is note-worthy that 

 the weaker solution is the most rapid in its effect. A simpler process of 

 photographic etching is inconceivable. If it answer as perfectly, and if its 

 range be as comprehensive as is here stated, the great question of sun- 

 engraving is in a fair way of settlement, not settled, however, as has been 

 too hastily assumed. The process here described is etching simply; and we 

 fear, from the description, is too superficial to produce many perfect impres- 

 sions. However, if it go no further, it is an extremely beautiful extension of 

 heliography, and to us it seems to lay the foundation for a more satisfactory 

 result than has } T et been achieved. London Literary Gazette. 



The process of Herr Pretsch, which excited so much attention a year ago 



