April 13, 1888.] 



SCIENTIFIC NEAVS. 



357 



which in appearance are not unlike roasted coffee. The 

 natives of Java use them as a tonic and to arrest the 

 spitting of blood. The shrub is the only known species, and 

 grows in the mountain districts of Java and Formosa. 

 Dr. Horsfield says the natives use the seeds as an anti- 

 dote to any poison, and Leschenault says the powdered 

 fruits mixed with food prevents diseases. Mixed with 

 lemon-juice, they are also applied externally to wounds. 



Mr. Christy also sent samples of chocolate made from 

 the seeds of Cola acuminata^ and which he had ascer- 

 tained to be useful in certain forms of indigestion. 



MODERN PHOTOGRAPHIC ENGRAVING 

 AND PRINTING.— II. 



A Paper by Mr. George S. Waterlow, read at the 



Conference of the Camera Club. 



( Continued from page 326.^ 



There are many details which are necessary to successful 

 etching, but those now given will be sufficient to convey 

 to you generally the method of making the zinc plate for 

 the typographic block. After etching, there only remains 

 the trimming of the zinc, a little touching up, and 

 mounting it on a block of mahogany of exact thickness to 

 render it type-high, and it is now ready for insertion with 

 type in the printer's forme. 



From a properly-etched plate, hundreds of thousands 

 of prints may be obtained, or it may be electrotyped or 

 stereotyped and multiplied indefinitely. 



The next process on my list is engraving on copper in 

 intaglio. There are many names now in vogue for this 

 beautiful process, but it is best known by that of " Pho- 

 togravure." The difference between an intaglio and 

 relief engraving is implied by their names. In the one 

 we are now to deal with the lines or ink-carrying parts 

 of the plate, are sunk below the surface, instead of being 

 highest, as in the case of relief or type blocks. 



The great advantage of this process in point of beauty 

 over that of zinc surface blocks is, that we are able to 

 reproduce a drawing which is made with washes instead 

 of lines, a painting, or a photographic portrait or land- 

 scape ; but for commercial and general purposes it is not 

 so useful as type-blocks, because of the expense and 

 slowness of the printing. The ink has to be rubbed into 

 the lines and hollows of the plate, and the surface 

 perfectly cleaned for every impression, 

 s There are many methods ot producing these plates by 

 the aid of photography, but details of most of them are 

 either unpublished or are worked as secret processes. 1 

 shall, however, be able to indicate the method to you in 

 general terms. 



Instead of a negative, a positive on glass is required 

 for the first operation. This positive must contain every 

 gradation of tone in the original, and be as nearly perfect 

 as possible. Next, a solution ot gelatine, sensitised with 

 bichromate of potash, and containing a graining matter, 

 is thickly spread over a glass plate and dried ; when 

 dry it is stripped from the glass and placed with the 

 glass positive in a printing-frame and strongly printed. 

 After printing, the film of gelatine is attached to a metal 

 plate, and developed with water like a carbon print ; 

 when sufficiently developed, the film is allowed fo dry ; 



* See page 157 



on examination it will be found that the picture appears 

 in the form of little elevations and depressions over the 

 surface, the blacks being thehighest and the whites almost 

 bare metal, each gradation of tone in the positive being 

 a different elevation in the gelatine. This film may now 

 be brushed over with powdered graphite and placed in 

 the electrotyping bath, and a printing copper-plate 

 produced. 



Another method is to sensitise the surface of the 

 copper itself with the same grained bichromatised gela- 

 tine, expose it under a positive, and bite through with 

 " perchloride of iron ; " the etching fluid passes through 

 the parts protected from light by the positive, and etches 

 the copper in exact proportion to the solubility or 

 insolubility of the exposed gelatine surface. This is the 

 simplest of the many processes, and it answers well for a 

 small number of printings, but it is not deep or strong 

 enough for much hard work. 



I now pass to a different photo-mechanical printing — 

 i.e., Woodbury printing. 



This ingenious process is, like the others, founded on 

 the peculiar oxidising action of gelatine in the presence 

 of bichromate of potash ; the process has now been 

 before the world so long that most of you will be familiar, 

 if not with the actual process, at least with its production. 

 Great credit is due to the originator of this admirable 

 process, and for the complete and practical issue to which 

 he brought it. 



Gelatine with bichromate of potash is spread upon 

 levelled plates of glass, dried, and stripped ; the 

 film thus obtained is exposed to light under an ordinary 

 negative ; the film is then cemented, face down on glass, 

 and washed for several hours with hot water. After 

 being allowed to dry, the film is again stripped from its 

 support, and presents the appearance of a delicate piece 

 of silk, with an exquisitely-modelled picture in fine 

 relief. Its texture, delicate and fragile as it may appear, 

 possesses extraordinary strength, and in the next opera- 

 tion is subjected to a pressure of hundreds of tons in con- 

 tact with the hardest steel, and comes out of the ordeal 

 unharmed, and ready to be used again and again without 

 injury. The printing-plate or block is made of hard, 

 rolled lead, and is placed face down on this gelatine film 

 or relief on a steel block in a powerful hydraulic press, 

 where it receives immense pressure — often that of 500 

 tons. The raised image on the gelatine is thus forced 

 into the lead plate or block, and gives a mould ready for 

 the special printing it is to receive. 



The printing is rather difficult to explain. The lead mould 

 is laid on a plastic cushion on the bed of a small press, 

 and its face brought up almost level, but not quite, with 

 a sheet of plate-glass cemented to an iron lid (or platen) 

 hinged on the top of the press ; the hinged lid is lifted 

 by the printer, and he pours on the mould warm 

 ink made of gelatine and colouring matter. A sheet 01 

 prepared water-proofed paper is laid on the pool of ink, 

 and the lid shut down and clipped by a lever motion ; this 

 spreads out the gelatine ink over the entire mould, 

 squeegees it away from the highest parts, or whites, and 

 leaves it only in the hollows, or dark and toned parts of 

 the picture. In a few minutes the warm layer of ink 

 cools and sets, and on the press being opened, the sheet 

 of paper with the ink from the mould adhering to it is 

 carefully removed. This is the Woodbury print, and 

 now only requires drying and fixing in alum to make it 

 the finished permanent photograph which is so familiar 

 to all. 



