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274 THE ADVANCE OF PHOTOGRAPHY
to revolve rapidly. The acid can be removed from the bath by means of the tap seen beneath the table. The whole operation of etching by this process takes only about two minutes, and it will be found that the plates have been etched quite evenly with a good direct downward etch. It is worth noting that there are no metallic parts of the machine in contact with the acid.
Scientific Diagrams — Line Drawings. — For the reproduction of scientific dragrams, and all other drawings in which the picture is built up of distinct lines and dots, it is not necessary to use the half-tone screen. Such illustrations are known as line drawings. Since this is so, a word or two on the preparation of such copy will be useful. The paper used should have a pure white, smooth surface, so that the amount of reflection may be just correct. As these drawings have often to be sent through the post, it is advisable to use cards of medium thickness so as to avoid errors, etc., due to doubling up the copy. It is, of course, by this time understood that some colours can be photographed with much shorter exposure than others, hence whatever colour is adopted should be used throughout the drawing. The best substance to use for drawing the lines is Chinese ink ; this gives a good black line which does not increase the difficulty of the photographer by giving an undue amount of reflection. If it is wished to shade in any part of the drawing, this must be done by strong fine lines which must always be kept quite distinct from one another. Should these lines be placed so close together that the amount of white shown is very small, then, if as is usually the case, a reduced photograph of the copy is taken, a solid black tone will be the result. When dotted lines are used, each dot must be separate and distinct, while again in cross-hatching, the lines when running in the same direction must be carefully drawn so as to be separate throughout their entire length. On this account they must not be drawn too closely to