IM 



UNDULATORY FORCES. LIGHT PHOTOGRAPHY. [pRimsu'b PROCI 



a CMC, the quantity of ink applied should be as small 

 as possible ; and to enable a light but evou coating to 

 be laid on it, must bo thin. As a consequence of the 

 small quantity <>f ink used, the transfer must be made 

 mi a MIIC... tli plate by the anastatic process ; because, to 

 make a successful transfer to a grained plate or stone, a 

 larger quantity of ink is necessary. On the otlu-r han.l, 

 as impressions taken from a grained plate or from stone, 

 are, as a rule, better than those from a smooth plate, 

 as a larger number of good impressions can be 

 striu-k ..;! if tin- subject photographed be so open that 

 there does not appear to be any likelihood of the lines 

 coalescing in the water, it is better to apply the ink in 

 greater pro|Hirtion to the print, as a certain quantity is 

 indispensable for the employment of the latter mode of 

 transfer. I5y the application of this process to the re- 

 ilurtion of the plans of the Ordnance survey, an 

 immense saving of time is effected ; while the total 

 money saving will amount to no less a sum than 35,000 

 on the cost of the survey." 



Having thus given our readers the above account 

 of the photozincographic process, we shall now pro- 

 ceed to describe that of M. Paul Pretcsh ; which, as 

 already observed, we have repeatedly examined in all 

 its details. 



M. PAUL PRETCSH'S PROCESS FOR 

 ENGRAVING BY LIGHT AND ELECTRICITY. 



PERHAPS one of the most ingenious and successful 

 inventions which has yet been made in attempting to 

 adapt photography to the purposes of the engraver, is 

 that invented by M. Paul Pretcsh, formerly manager of 

 the Imperial Press at Vienna. That gentleman cour- 

 teously explained the entire process to us ; and we shall 

 give our readers a general outline of it. 



The inventor avails himself of the fact, that animal 

 matter, combined with bichromate of potass, is pecu- 

 liarly acted on by light, and becomes thereby consider- 

 ably hardened. A mixture of good glue or gelatine is 

 made with a solution of bichromate of potass, aided by 

 the application of heat. It is of such a consistency 

 as to become solid when cool. Whilst in the liquid 

 state, solutions of nitrate of silver and iodide of potas- 

 sium are added to it ; and after the whole has been well 

 incorporated together, the liquid is poured on to a clean 

 glass plate, and allowed to cool in a dark place. 



Presuming that an engraving is to be copied, it is to 

 be placed, face downwards, on one of these prepared 

 plates, and pressed closely on the glued surface. This 

 is best effected by the copying frame referred to, and 

 illustrated, in a previous page.* It is then to be ex- 

 posed in the usual manner, varying from a few hours -in 

 direct sunlight, to a day or two in diffused light. On 

 removing the plate from the frame, it will present no 

 great visible change, or, at all events, its surface will 

 still bo perfectly level, even if some effect of shadowed 

 outlines may be perceived. It is then to be immersed 

 in water, or, as we believe the inventor recommends, a 

 solution of common salt in water. An immediate effect 

 takes place : those parts which have been exposed to 

 solar action retain their previous condition; whilst those 

 which have been shaded by the dork part of the en- 

 graving from the action of light, immediately rise up 

 beyond the ordinary level, and form a series of raited 

 lines, exactly resembling the shaded lines in the ori- 

 ginal The glue surface, therefore, assumes exactly the 

 opposite condition of that of an engraved plate in which 

 the shaded portions are hallow lines. The plate is then 

 dried ; and when again each part has become firm, a cast 

 is taken from it by means of any suitable substance, 

 such as plaster of Paris, which, of course, produces an 

 intaglio impression. From this n cast is taken by 

 means of gutta-percha, <fec., which becomes the mould, 

 from which a solid engraved copper-plate may be ob- 



Fur this purpose the electrotype process is 

 employed; to explain winch, we may observe, that a 

 current of voltaic electricity passing from one pole of a 



See fntt, p. Ii4. 



battery through a metallic solution, conveys at the same 

 time portions of that pole to the other extremity or 

 terminal of the battery. A galvanic battery consists of 

 a sheet of zinc, and one of copper, silver, or platiua. 

 To each metal a wire is attached, and the two sheets of 

 metal are placed in a jar, into which acid and water are 

 poured. So long as the two metals do not touch each 

 other, or the wires attached to them are apart, no action 

 takes place in the battery. But on contact being made, 

 then a current of electricity passes from the zinc, through 

 the liquid to the copper, and thence by the wire, back 

 to the zinc. If, however, the two wires are placed in 

 another vessel containing a solution, say of sulphate of 

 copper, then the electncity will pass from the wire 

 attached to the copper-plate, through the nolution, to the 

 wire proceeding from the zinc plate, and the latter wire 

 will receive continually the metal from the opposite 

 one, till it is entirely dissolved away. If the gutta- 

 percha mould is first blackleaded carefully so as to 

 moke it a conductor of electricity, and it is then 

 attached to the wire from the zinc end of the battery, 

 whilst a copper-plate is hung from that wire proceeding 

 from the copper end of the battery both being im- 

 mersed in a jar containing a solution of sulphate of 

 copper, and dilute sulphuric acid the mould will bo 

 completely covered with the metal transferred from the 

 plate by the electric action ; and in the course of twenty- 

 four hours' action, a complete copper copy will be 

 produced. This is next gently removed from the face of 

 the mould ; its edges are trimmed ; and it may then bo 

 employed in the printing-press, in exactly the same 

 manner as an ordinary engraved copper-plate. 



It will be perceived that, by this curious process, a 

 complete and perfect engraved copper-plate may ba 

 produced by the sole action of light and electricity ; and 

 viewed in a philosophical point of view, it presents the 

 most astonishing application of two forces with which 

 we are acquainted. 



For the sake of simplicity, we have only supposed the 

 case of the copying of an engraving. Photographs, <tc., 

 can, however, be easily reproduced by the same means ; 

 and thus an exact copy of any object can be almost 

 indefinitely produced. The inventor, however, finds, 

 that, owing to the softness of copper deposited by the 

 electrotype process, not more than six or eight hundred 

 copies can be printed from one plate. Any number of 

 plates, however, may be produced from the original mould. 

 We have seen n'vmerous specimens of engravings so 

 procured; and have no hesitation in pronouncing them 

 to be of the greatest accuracy and perfection in their 

 detail and general effect. The ingenious inventor has 

 applied the process to a variety of purposes ; amongst 

 which we may mention that of producing engravings 

 from personal photographs; by means of which, linen 

 or cotton may be marked with the likeness of the 

 owner, intead of by the ordinary plan of marking-ink. 

 Our readers of the fair sex will instantly appreciate this 

 application, because it prevents that constant inter- 

 change of the ownership of property, to which the 

 laundry-women of our large towns are so prone. Simi- 

 larity of initials is common enough ; but a photograph 

 on wearing apparel at once stamps an unerring title as 

 to the proper right of possession. 



We are not aware that this invention has received that 

 encouragement which its value deserves. Several illus- 

 trated works, however, have been published, in which 

 the cuts have been produced by the process. Its value 

 greatly consists in the accurate reproduction of natural 

 scenes, <tc., represented by it. 



STEREOSCOPIC PHOTOGRAPHY. 



TUB stereoscope is so well known as not to require any 

 particular description on our part. It gives to the eye 

 the effect of solidity from two pictures, placed in a 

 pro] KT position, the images overlapping each other. We 

 have already entered into an exposition of the prin- 

 ciples of tho laws of vision ;t and our remarks on the 

 t Pee ante, pp. 44 and 49. 



