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136 SCIOPTICON MANUAL. breathing on and thoroughly rubbing it with tissue- paper is sufficient for the cleaning. Of course the fin- gers must not touch the surface of the glass after it is cleaned. The next step is to prepare the surface of the glass for the drawing. If the glass (as is usual with this kind) is slightly curved, the concave side should invariably be the surface to receive the preparation. This preparation is as follows: " The plate to be dusted and gently warm- ed; then flow the surface with No. 1 varnish, and drain into the bottle from the corner of the glass. When this is dry, flow with No. 2 in the same manner, and after- wards dry with gentle heat. Of course until the var- nish is dry the corner from which the varnish was drained should be kept down. Should dust find its way into the varnish, it can be filtered." Having prepared the surface, it should not be soiled by handling (always take the glass by the edge between the fingers). A sketch can now be made on the surface with a good hard black-lead pencil, either as an original drawing, with the glass on the easel, or by placing the glass on an engraving or other picture, and tracing the outline. When this sketch or outline is finished, strong and black lines and marks can be made with the " liquid India-ink " in a fine pen, gently used so as not to make the lines too coarse and heavy. With the pen, too, lines can be ruled or letters or figures added. After the drawing is finished and dry, any little corrections or scratches can be made with the pointed eraser. The easel should be so placed that the light falling on the white tablet will be reflected to the eye of the artist through the inclined glass on which the picture rests, and consequently the picture will be illuminated by transmitted light. The easel may stand on a table placed against a win-