9S J. Waterliouse — The Application of Pliotograpjly [No. 2, 



The plate is then replaced in the drying hox and when dry is ready for 

 exposure to light in the usual way ; but it will be found desirable, in order 

 to secure perfect contact, to transfer the negative film on to the gelatine 

 surface in a bath of alcohol as before described. 



Formic acid varies in strength and other properties, and if it should be 

 found that the films made by the above formula are too soft, the plates 

 may be kept a few days before printing. The addition of a very small 

 quantity of tartaric acid (about /q of a part) will improve the films in this 

 respect, and so will the cautious addition of some hardening agent, such as 

 chrome alum, glycerine, glucose, honey, &c. 



The printing operations are the same as for the plates already described, 

 but the use of glue rollers and vertical pressure will be found advantageous. 

 The thin films have been found to stand the wear and tear of printing well 

 and to have no tendency to chip or tear away from the plates. 



In all cases where the photographic image is impressed directly on the 

 printing surface, a reversed negative must be used, as before explained, and 

 these are sometimes rather troublesome to produce. 1 have lately tried 

 whether the use of this reversed negative could not be dispensed with in 

 the photocollotype process, by taking the negative in the usual way direct 

 on to the thick ground glass plate and then, while still wet and without 

 varnishing, coating this negative with a thin layer of any of the foregoing 

 mixtures of gelatine, either with or without bichromate. When the sensitive 

 gelatine coating is dry, it is exposed to light through the negative on the 

 under side and allowed to print well through the film. This plan was 

 found to have many conveniences to recommend it, and to answer very well 

 for subjects in line, but not for half-tones. For map-work it has the 

 undoubted advantages of perfect accuracy of scale and the greatest possible 

 sharpness of the image. 



The foregoing descriptions will give an idea of these interesting 

 processes which are now being very largely used for producing photo- 

 graphic prints of all kinds, though, I believe, the successful working 

 of them still presents some difticulties, even in better climates than India. 

 Against their employment for map-work on the large scale there will, how- 

 ever, always remain the impossibility of joining up several sections of a 

 large map on the printing surface, the difficulty of reproducing the finest 

 tints of a shaded map with a perfectly clean white ground, and, above all, 

 the difficulty of making additions and corrections on the plates. 



The accompanying specimen of a reproduction of an old map of Bengal 

 will give an idea of what may be done by the process described at page 93. 



