1878.] to tJie Beproduction of Maps and Plans. 67 



versed negatives for collotype and other purposes, it is a good plan, 

 when possible, to have an arrangement for laying the plan horizontally 

 under the lens at any convenient distance from it. 



The whole of the apparatus connected with the camera and plan-board 

 must be rigid and firmly fixed, so as to be free from vibration. The 

 slightest vibration is sufficient to destroy the perfect sharpness of the image. 

 In the glass-house attached to the Photographic Branch of the Surveyor 

 General's Office here, I have endeavoured, and I think with success, to over- 

 come all vibration caused by carriages passing in the street close by, by 

 dividing the floor of the camera-room into isolated blocks resting on a bed 

 of sand, so that each camera shall stand by itself on a block isolated from 

 adioining blocks and from the walls and floor of the building The plan- 

 boards are fixed on a separate wall quite isolated from the walls of the 

 building. 



Plans may be copied either in the open air or under shelter — coloured 

 and old stained manuscripts, maps or drawings are better copied in full 

 sunlight. The glass-house I have constructed at the Surveyor General's 

 Office faces the south and is glazed with ground glass, so that a strong 

 diffused light may be thrown upon the plan-boards. When circumstances 

 permit, it is well to have the camera and plan-board mounted on a firm 

 stand working on a pivot, so that, as the day wears on, the position of the 

 plan-boards may be changed so as always to face the sun. I adopted this 

 arrangement at the Trigonometrical Survey Office, Dehra Dun, and I 

 believe it has many advantages over the fixed glass-house rendered necessary 

 in Calcutta by the constant wind and dust, and the greater necessity of 

 being able to carry on work without interruption at all times of the year. 



The negatives of maps &c., drawn in line only, for reproduction by 

 photozincography, are taken by the ordinary wet collodion process with iron 

 development, modified so as to secure the greatest transparence in the lines 

 and density of the ground ; but as the ordinary wet collodion process by 

 itself will not give all the intensity required to produce an almost opaque 

 ground, it is obtained by intensifying the negative in the usual way with 

 pyrogallic acid and silver, after fixing ; then treating it with a saturated 

 solution of bichloride of mercury till the film becomes white, and finally 

 applying a dilute solution of hydrosulphate of ammonia, which instantly 

 changes the colour of the film to a dense black or brown throughout. The 

 negative is afterwards varnished with a resinous varnish, or flowed over, while 

 wet, with a solution of gum or gelatine and allowed to dry. All defects, pin- 

 marks &c., are then stopped out with Indian ink or black varnish. In 

 taking the large negatives on plates 32 x 24, that we are now producing for 

 copying the maps of the Cadastral Surveys, it has been fomid that the first 

 intensification may be produced by washing the plate after the first develop- 



