With a view to avoiding confusion in the treatment of our subject, we here refer to a few processes which do not frequently come within ordinary practice, but which, nevertheless, should be understood, on account of their occasional usefulness. Some of them might have been included in a previous chapter, but it has been thought best to allot them a separate one.

130. Tracing Photographs and other subjects of indistinct outline.—It often happens that the lithographer requires an outline tracing of a subject which, though tolerably clear in itself, becomes very confused when seen through the ordinary tracing-paper. There are several methods of overcoming this difficulty.

I.—Very transparent tracing-paper may be made by coating fine tissue-paper with crystal varnish. Coat it on one side, and let it dry, and then coat it on the other. The varnish may be made by adding spirits of turpentine to Canada balsam until thin enough to be used with a varnish-brush, the solution being effected with a gentle heat.

II—French tracing-paper, papier végétal, may be oiled, to render it more transparent; but care must be taken to avoid bringing the oiled surface in contact with the stone or transfer-paper.

III.—Ordinary tracing or other thin paper may be wetted with spirits of turpentine or benzoline, and the wetted side applied to the glass or paper photograph. The tracing may then be made in pencil The liquid will soon evaporate; but if the paper becomes opaque in consequence, a corner may be lifted and another drop of the fluid introduced, which will instantly restore the transparency.