1878.] to the Reproduction of Maps and Plans. 05 



" Iron mould and ink spots may be taken out with a solution of oxalic 

 acid or salts of sorrel." 



When tracings are made on paper or vellum cloth to be reproduced 

 without the aid of the camera, special care must be taken to keep the back 

 of the drawing clean, and to choose paper or cloth free from stains and of 

 as even a texture as possible. 



Originals drawn on rough paper may be smoothed in a copper plate 

 press, and, if dirty, should be carefully cleaned with india-rubber or bread. 



III. The Peodijction of the Negative. 



After the due preparation of the original, the production of the 

 negative is a point of the utmost importance, and may well be considered 

 by itself before proceeding to the consideration of the various processes 

 of photographic printing. 



In order to obtain the most satisfactory results for photolithography, 

 photozincography, or any other process specially applicable to line subjects, 

 the negative must be perfectly sharp all over, free from distortion and 

 possess the greatest amount of contrast between the lines and the ground. 

 If care is taken to produce good negatives from suitable originals, results 

 may be obtained which will compare with ordinary lithographs and 

 engravings for sharpness and delicacy. The difference in the results of 

 working with good negatives or bad ones is incredible ; with a good nega- 

 tive from a good original every thing works well, but with a bad negative 

 from a faulty original all kinds of difficulties may be encountered, and the 

 attainment of a passable result is almost a matter of chance. 



The first thing is to arrange the plan so that it may be copied without 

 any distortion and be quite sharp all over. 



To ensure freedom from distortion, the lens employed must give an 

 image quite free from all curvature of the marginal lines of a rectangle. 

 In practice the most suitable forms have been found to, be the ' Bectilinear^ 

 of Dallmeyer ; the * Doublet' of Eoss ; ' Aplanatic' of Steinheil and others 

 on the same principle. The lenses known as triple combinations are also 

 good. In the Surveyor General's Office, Calcutta, Dallmeyer's Kapid Eecti- 

 linears are used and found to answer well. The lens should be worked well 

 within its power, so as to use the most central rays ; and to secure the 

 sharpness of the image all over the plate, a small stop or diaphragm should 

 be used. 



The plan must be placed so as to be evenly illuminated by a good 

 strong light falling as horizontally as possible, in order to avoid shadows 

 being thrown by the grain of the paper, and thus diminishing the even 

 opacity of the ground of the negative. 



