68 J. Waterbouse — The Ap2->licnfion of Fhofogrnpliy [No. 2, 



ment and applying a weak solution of nitrate of silver followed by a second 

 application of tbe iron developer. 



Otber metbods of obtaining tbe extra density required for these nega- 

 tives bave been proposed and are in use,* but, notwithstanding several in- 

 conveniences arising from the use of bichloride of mercury and hydrosulphate 

 of ammonia, the above appears to be the best and most certain when working 

 on the large scale. 



When maps are not drawn entirely in pen and ink but have the hills 

 brush shaded, and it is desired to reproduce them by the collotype or engra- 

 ving processes, great care and skill are required on the part of the photo- 

 grapher to get the ground of the negative dense enough to give a perfectly 

 clean impression in the white parts of the map, and at the same time pre- 

 vent the grain of the paper from showing and give the faintest tints of 

 the shading their proper value. Coloured maps also give a good deal of 

 trouble, and when allowable the colour should be washed off as much as 

 possible before the negatives are taken. Colours may sometimes be removed 

 by chemical means, but there is risk of injury to the original. 



In many of the processes about to be described it is necessary to 

 use a negative which instead of giving an image reading the same way 

 as the original shall give it reversed as to right and left. There are several 

 methods of obtaining these reversed negatives. 



(1.) By coating the original unreversed negative with a thick transfer 

 collodion, t or a layer of gelatine, and then stripping off the film and, either 

 laying it down again on a sheet of glass in a reversed position, or using it 

 as a film negative which may be used for giving both reversed or unreversed 

 images. This method is practical and useful, but is not suitable for map 

 work on account of the liability to contraction and distortion of the image. 

 There is also difiiculty in keeping the film negatives flat, and they have 

 been found to become brittle and perish very soon in this climate. 



(2.) By turning the sensitive plate in the camera, so that the light 

 acts, through the glass, on the back surface of the film. This method is also 

 practical, simple and useful, but requires care in the selection of glass 

 plates free from scratches, &c., and in wiping the back of the plate before 

 it is put in the camera. It is used in the Belgian Topographical Bureau 

 with dry tannin plates, but I have not found it suitable for reproducing 

 very fine map-work on wet plates. 



(3.) By placing a reversing mirror or prism in front of the lens — the 



image thus passes through the lens reversed and is impressed directly on 



the sensitive plate. This is one of the simplest and most effectual of all 



methods. If a mirror is used it should be one silvered by depositing silver 



* See Abney, Instruction in Photography^ p. 22. 



t See the same work, p. I'SO, 



