1878.] to the Beproduction of Maps and Plans 71 



of the print while the back surface of the sensitive paper is shielded from 

 light. The exposure to light lasts for some minutes, by which the parts 

 unprotected by the denser parts of the negative are darkened more or less, 

 according to its translucency, while the parts entirely shielded from the 

 light remain quite white. When the action of the light is judged sufficient, 

 the sensitive paper is removed from the frame in a dark place, and must 

 then undergo an operation oi fixing to remove the unchanged salts of silver, 

 which would cause the print to darken unless carefully protected from the 

 light. This is effected by steeping the print for a short time in a solution 

 of hyposulphite of soda ; but before the print undergoes this indispensable 

 operation it is usual to place it in a solution of chloride of gold, by which 

 part of the reduced silver forming the image is replaced by a film of gold 

 and the print takes a more agreeable tone, also becoming more permanent 

 than it would be if this operation, called 'toning,' were omitted. It is, 

 however, impossible to ensure perfect permanency of these prints, by reason 

 of the sulphur contained in the albumen or in traces of sulphur salts formed 

 by the decomposition of the hyposulphite of soda, and left in the print after 

 even the most careful washing, slowly acting on the reduced silver forming 

 the image and converting it into a sulphide, by which the tone and bril- 

 liancy of the picture are lost, and the lighter shades appear to fade away 

 entirely. It should, however, be stated that prints prepared on plain, or 

 unalbumenised, paper are more permanent than the albumenised prints, 

 though not so brilliant, sharp and delicate ; and they have the further 

 advantage of being less liable to shrinkage and distortion than the albu- 

 menised prints, and are thus more suitable for the reproduction of maps 

 where accuracy of scale is a desideratum. 



It is evident that owing to the expensiveness of the materials used in 

 producing these prints and their want of permanency, together with the slow 

 rate at which they can be produced, this process is almost useless for the 

 reproduction of maps in large numbers ; and, in fact, its use in cartography 

 is limited to making copies of special maps for immediate reference or 

 temporary purposes, and as guides for engravers or lithographers in 

 preparing compilations from maps on a lai^er scale. Silver prints have also 

 been used by engravers to obtain a correct tracing on the waxed surface of 

 their copper plates, but unless these prints are prepared with great care 

 they are open to the objection of becoming distorted and untrue to scale by 

 the contractions and expansions caused by the successive wasliings they 

 have to undergo. Prints on plain paper are better for this purjiose 

 than those on albumenised paper, and prints on paper containing a largo 

 proportion of resin in the size are better still. 



In the English Ordnance Survey the topographers arc furnished 

 with silver-print reductions from the large scale outline survey, on which 



