1878.] to tlie Bcproduction of Maps and Plans. 107 



light under a photographic cliche, and then plunged into hot water, the 

 parts acted on hy light being insoluble will remain on the support in differ- 

 ent degrees of relief according to the intensity of the light, while the 

 unexposed parts will be washed away. An image in high relief formed of 

 hard and insoluble gelatine will thus be obtained, from which a cast or 

 electrotype in intaglio may be made. 



If, however, instead of using hot water, the j)late be plunged into cold 

 water, the gelatine will be found to absorb water and swell up in the parts 

 protected from the light, while in the parts acted on by the light it will only 

 slightly absorb the water, and these parts will thus form hollows. The 

 power of absorbing water will also be found to be in exact proportion to the 

 degree of protection from light. In this case, an image in low relief 

 is obtained which may also be moulded from, or electrotyped. 



Upon these two principles several processes of j)roducing printing- plates 

 both for copper-plate and letter-press printing have been founded with 

 more or less success. 



The first process of the kind was Paul Pretsch's ' Photogalvanography', 

 invented in 1854^. He appears at first to have obtained his plates by coat- 

 ing a glass with gelatine and bichromate, exposing to light and then wash- 

 ing away the soluble gelatine and taking a mould of the resulting relief, 

 in gutta-percha, from which an electrotype was made in the usual manner.* 



This process gave fair results both in line and hl!f-tone, but, owing to 

 the washing away of the soluble gelatine being effected on the side of the 

 film exposed to light, the plates were defective and required a good deal of 

 touching up by skilled engravers, which vastly increased the expense of their 

 production. The process failed as a commercial speculation. 



Almost immediately afterwards, in 1855, Poitevin published methods 

 of obtaining plates from the gelatine reliefs obtained by swelling the sen- 

 sitive films in cold water. Plaster casts were either made from them or the 

 gelatine surface itself was metallised and electrotyped in the usual way.f 



This method produced tolerable results, though the prints were always 

 somewhat coarse, owing to the fact that swollen gelatine will not give the 

 same sharpness as when dry. 



Both these processes were more or less unsuitable for reproducing sub- 

 jects in half-tone — Pretsch's because in the process of washing away the 

 soluble gelatine, the lighter half-tones w^ere liable to be lost — Poitevin's be- 

 cause of the difficulty of keeping the swelling in proper relative proj^ortion, 

 owing to more absorbent parts of the film lying underneath less absorbent 

 parts. Both processes also failed to give the necessary ' grain,' without 

 which the proper inking of the engraved plates could not be effected. 



* Sco ' Journal of the Thotorjraiihic Society of London,'' Vol. Ill, p. o8. 

 t See ' Traiid de I'lmpression photographique sans sets d^argcnt,' p. 49. 



