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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 99
black points more or less near together. The granular structure of the stone does not allow such delicacy as may be attained by photography.
In one branch photo-lithography surpasses all other reproducing arts — that is, in producing copies of maps which have been drawn by hand. The preparation of geographical maps requires much time and care. The outlines of mountains, rivers, and countries must be executed with the greatest exactitude, corresponding to the measurement. Frequently draughtsmen and engravers are employed for the various details, and though working conscientiously, inaccuracies are unavoidable, and make corrections necessary. All this takes time and trouble. If the object is now to make an enlarged or diminished copy of an existing map, the same difficulties occur, and the diminishing is especially troublesome. The pantograph is a useful aid here, but does not exclude need for attention in the draughtsman. In this respect photography is invaluable as an aid to map-making. With very great ease, enlarged or diminished copies of an original may be prepared as negatives by photography ; in a few hours this is copied on stone, and within a day photo-lithography can throw off thousands of enlarged, diminished or original sized copies.
If it were wished to make a lithographic stone drawing by hand, several days would be necessar}r, and it would be far less exact. No photographic printing process is as rapid as photo-lithography, and it has in consequence been much used in map-making, especially where a large number of copies are required. In the Franco-Prussian war, the advancing troops needed, before all things, maps of the territory to be occupied. But there was not a sufficient supply of maps of France to provide whole army corps. It is impossible to keep a stock of such maps, as no one can tell where a campaign will take place. Photo-lithography was here an im