1878.] to the Bejyroihiction of Maps and Flans. 8T 



and, indeed, is difficult to remove without grinding the stone down to some 

 depth. 



This process requires care in adjusting the amount of moisture to be 

 applied to soften the gum, so that it may not be squeezed under the lines 

 and block them up, and it has not, I believe, come into general use. 



There are two disadvantages Avhich militate against the employment of 

 the transfer processes of photolithography for the finer and better class of 

 maps. The first, is the difficulty of obtaining reproductions perfectly true 

 to scale, owing to the unequal expansion of the transfer paper in the various 

 washings and squeezings it has to undergo. Although this unequal expansion 

 and contraction is very slight, and for most practical purposes may be disre- 

 garded, it has greatly hindered the more universal adoption of this valuable 

 method for the reproduction of the official maps in England and foreign 

 countries. 



Mr. Eodriguez, of Lisbon, has, however, lately introduced an improve- 

 ment into the transfer process with the object of doing away with the 

 possibility of stretching in the course of any of the operations.* Instead 

 of using paper as the support of the coating of gelatine on which the pho- 

 tographic image is impressed, he uses a sheet of tinfoil about the thickness 

 of thin paper. This is first smoothed on a very finely grained lithographic 

 stone and then laid down quite flat on a sheet of zinc. After being 

 cleaned with alkali and well washed, the tinfoil is brushed over with a 

 solution of gelatine and bichromate, dried rapidly, and is then ready to be 

 exposed under a negative in the usual manner. To ink the print, the sheet 

 of tin is first plunged into water, and then carefully laid down wet on a litho- 

 graphic stone so as to avoid folds, the gelatine side being uppermost. The 

 film is then inked in with a roller. After the first inking in the print is 

 left for about a couple of hours and is then inked in again and afterwards 

 washed with a sponge and water. It may then be lifted off the stone and 

 dried. The operations of transfer are the same as usual. 



The second disadvantage of the transfer methods is the almost un- 

 avoidable spreading of the lines under the operation of transferring, which 

 makes a photolithographed map look heavy and unsightly compared with a 

 lithographed one. This defect may, however, be diminished very much 

 by skilful manipulation and taking care to have as thin a coating as pos- 

 sible of gelatine on the paper, and to use a good hard transfer ink in small 

 quantity. With these precautions and with a suitable original, results may 

 be obtained, from photolithographic transfers which will well compare with 

 ordinary lithography, or even engraving, in sharpness and delicacy. 



These special defects of the transfer metliods may be in great part 

 •bviatcd by impressing the photographic image direct on the stone, as origi- 

 * ' Brilish Journal of Thotoijraphij: Vol. XXV, p. 232. 



