1878.] to the Reproduction of Maps and Flans. 117 



by the fact that to produce a successful printing block the surface of 

 all the lines or dots which receive the ink must be very nearly on one 

 uniform level, and therefore the moulding processes above described are 

 inapplicable. The second and greater difficulty is to obtain a suitable 

 grain to break up the continuous gradation of shade in the photograph. 



M. Rodriguez, of Lisbon, has proposed an ingenious method by which 

 promising results have been obtained. He makes a paste of sugar of milk, 

 or some other substance in powder soluble in nitric acid, with a little oil of 

 lavender and bitumen, and adds a sufficient quantity of it to a solution of 

 bitumen in turpentine. The metal plate is thinly coated with this in the 

 ordinary way, exposed to light and developed with turpentine. The plate 

 is then plunged into a bath of dilute nitric acid which gradually penetrates 

 the resinous coating and dissolves the substance used for forming the 

 grain, breaking up the preparation more or less according to the thickness 

 of the bitumen, and thus reproduces the half -tints of the originals.* 



In many processes of collotype the gelatine film presents a very 

 marked grain, which may be coarse or fine according to the composition 

 employed. It is probable, though I have not tried it nor, so far as I can 

 recollect, seen it proposed, that blocks showing very fair half-tone could 

 be obtained by taking a print from such a plate with a grain, transferring 

 it to zinc and then biting it in by a method similar to Gillot's already 

 described. Very great care would have to be taken in the successive 

 etchings to preserve the uniformity of surface and protect the finest 

 tints from being bitten too much. 



The prints in half-tone obtained by Mr. D. Dallas' process, known 

 as " Dallastint", appear to have been produced by some such method. 

 This, however, is only a conjecture on my part, because no details of the 

 process have been published. 



M. Rousselon has, I believe, obtained fair results by similar transfers 

 from his engraved plates, and it is probable also that a transfer to zinc 

 from one of the plates, prepared by the modification jDroposed by myself of 

 Geymet's photo-engraving process, bitten in in the same way, might also 

 answer the purpose, though the grain is perhaps scarcely strong enough. 



Details of several of the methods of photo-typography will be found 

 in Motteroz's " Essai sur les gravures chimiques en relief and Scherer's 

 " Lehrhuch der Ghemigrapliiey 



X. Miscellaneous Processes. 

 In addition to the processes by which photographic prints are obtained 

 directly by the aid of light, there are several ways in which photogra))hy 

 can be employed as a useful auxiliarj^ in obtaining correct tracings for tlie 

 * Sec ' Bulletin de la Soc. Franc, de rholoymphk;' pp 208, 2.'51. 

 1(5 



