It has been stated above that Bristol board is the best material to use for the making of drawings for line blocks; other materials may, however, be employed, although they are not so nice to work upon. For instance, it may be necessary to reproduce a map; this, as has already been mentioned, may be conveniently done by pinning over the map a sheet of pale blue tracing linen, and tracing the map on this with Indian ink. The fact that the linen is blue does not matter, for it will photograph as if it were white. Then again, many subjects may be of so complicated a nature as to be beyond the skill of the author to draw. In such a case a good plan is to take a photograph of the object and make a positive on smooth bromide paper, which need only be developed sufficiently far to give a print which just shows the features. The print, when dry, can then be worked on with fixed Indian ink. The finished drawing, when quite dry, may be immersed in any solution which will dissolve out the silver; a solution of iodine in potassium iodide answers sufficiently well. The print will turn very dark, but it must be allowed to remain in the bath until all the silver has dissolved; it is then removed, rinsed under the tap and placed in an ordinary fixing bath of hyposulphite of soda. All the colouration will be quickly removed so that the ink drawing will stand out well against the white paper. All that it now requires is a thorough washing in water; when dry it may be touched up and then placed under pressure in order to make it quite flat.[A] [Footnote A: The chief disadvantage of iodine solution is its slowness of action; the following methods are much quicker. (_a_) To a solution of 4 oz. of hyposulphite of soda in one pint of water, add a 10 per cent. aqueous solution of potassium ferricyanide until the mixture is lemon coloured. When the silver image has quite disappeared, wash the print thoroughly in water. Since the mixture does not keep, the ferricyanide solution should be added to the hyposulphite solution immediately before use. (_b_) Mix 125 ccs. of a 10 per cent. alcoholic solution of iodine with 21 ccs. of a 10 per cent. aqueous solution of potassium cyanide, add to the mixture 1 litre of water. When the image has disappeared, which will be in less than a minute, wash for five minutes in water and dry.] This method of drawing over photographic prints will often save a considerable amount of time. For instance, it may be desired to reproduce a consecutive series of drawings to illustrate the microscopic structure of the subject. The ordinary way of doing this is to make camera lucida drawings of the sections, which is a lengthy and tiresome process; a photograph of each section will take much less time and will give quite as good results.