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



nally proposed by Poitevin, but this plan has again other disadvantages of 

 its own which render it less suitable for map work than the transfer process. 

 It has, however, been used extensively, and very successfully, in the produc- 

 tion of the Belgian topographical maps on the scale of 1 : 20,000. 



In the process used for the Belgian maps, the stone is covered with a 

 very thin coating of a mixture of gelatine and bichromate of potash, rapidly 

 dried and exposed to light under a reversed negative, which is obtained by 

 reversing the position of a dry tannin plate in the camera and allowing the 

 light to act through the glass on the underside of the collodion film. A 

 thin coating of printing ink is then applied all over the stone with a roller, 

 and the surface is afterwards washed with warm water in which a little starch 

 has been dissolved. This gradually removes all the soluble parts of the 

 gelatine coating, leaving on the stone a clear image of the map. The stone 

 is then covered with gum and after drying and remaining for a short time 

 is ready for printing and capable of yielding 1500 good impressions.* 



For line-work zinc plates are also used and prepared in much the same 

 way. 



This process has undoubtedly some advantages as regards accuracy of 

 scale, and the quickness and cheapness of the operations, but on the other 

 hand it has disadvantages as regards the difficulty of securing perfect 

 contact between the stone and the negatives, the necessity for a reversed 

 negative, the prints being limited within a single negative and the incon- 

 veniences of working with heavy stones. 



Besides the foregoing, many methods of photolithography have been 

 proposed, but as for the most part they are only modifications of the 

 processes I have described, which are all good and may be considered typical, 

 it will be unnecessary for me to go further into details regarding them. 



VI. Photocollottpe. 

 The great defect of all the processes of photolithography described in 

 the last section is, that they can only be applied with advantage to the 

 reproduction of drawings or subjects in which the gradation of shade is 

 shown by lines or dots separated by white spaces of varying sizes and at 

 different intervals apart, as in line or stipple engravings and lithographs in 

 line or chalk. Even such drawings to be successfully reproduced must be 

 in a good bold open style and have all the lines or points composing them 

 of an equal and perfect blackness. In the many attempts that have been 

 made to reproduce photographs from nature by photolithography or photo- 

 engraving, or to copy paintings and brush-shaded drawings in which grada- 

 tion of shade is continuous, success, only partial at best, has been secured by 



* Maes and Hannot's ' Traite de Topographie, et de Reproduction des Cartes au 

 moyen de la Fhotographie ' / also Hannot's ' La Fhotographie dans les Armees, ' 



