Ill J. Waterhouse — The Application of Photography [No. 2, 



to be printed without deterioration of the plate, and the coating can easily 

 be removed and renewed whenever required. Details will be found in Ure's 

 ' Dictionary of Arts ^ Manufactures and Mines,'' article ENaEAYixa. 



IX. PnOTO-TYPOGEAPHT. 



The object of the photo-typographic processes is to obtain a surface 

 block by photographic agency, that may be set up with type in the same 

 way as woodcut, stereotyped or electrotyped blocks, and be printed in the 

 ordinary printing press. The process offers great advantages in the 

 rapidity with which the blocks may be made and printed off in large 

 numbers. Up to the present time no entirely satisfactory method has been 

 discovered for printing subjects in half-tones in this way, though Mr. Dun- 

 can Dallas has produced some very promising results. The processes are, 

 therefore, almost entirely limited to the reproduction of subjects in line 

 or dot alone. 



The operations in this branch of photographic reproduction are based 

 upon exactly the same principles as the photo-engraving processes just con- 

 sidered, and in some of them the only difference is the substitution of 

 a positive cliche for a negative, or vice versa. 



The existing processes may be divided into three classes : 



1st. Those in which a mould is made from a relief in swollen gelatine. 



2nd. Those in which the image is obtained in asphaltum or gelatine 

 on a metal plate and bitten in. 



3rd. Those in which an image in a waxy and resinous ink is obtained 

 by the methods described under the head of photozincography, then trans- 

 ferred to a metal plate and bitten in. 



Moulding Processes. — Of the first class several methods have been 

 introduced from time to time, but they are all on the same principle and are 

 modifications of Pretsch's and Poitevin's processes already described, differ- 

 ing, as a rule, merely in technicalities which being trade secrets have not 

 been fully published. 



The following method is a typical one. A glass plate or other suitable 

 surface is coated with a mixture of gelatine and bichromate of potash and 

 when dry exposed to light under a negative. After this, it is immersed in 

 cold water till the parts unaltered by the light, which represent the whites 

 of the original drawing, swell up to the required height, leaving the lines 

 quite sunk. The plate is then removed from the water and, the superfluous 

 moisture having been carefully blotted off, is ready to have a cast made 

 from it. 



This may be done in two ways first, by metallising the gelatine surface 

 either by means of plumbago or bronze powder, or by reducing silver upon 

 it by applying a solution of nitrate of silver followed by treatment with a 



