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CH. VIII] PREPARATION OF LANTERN SLIDES 205
diagram or table, etc., and then have a photographic lantern slide made.
In preparing these slides direct, a device of the artists of earlier times who painted lantern slides, is used. That is, the slide is cleaned carefully and then coated with a thin solution of some hard varnish or with gelatin (fig. 112, § 317). After the varnish has thoroughly dried one can use a pen or a brush upon the varnished surface with the same facility as upon paper. The hand-made slide is then mounted as usual and can, of course, be used indefinitely.
If they are for a special occasion — as in projecting election returns, games, etc., — the slides are used without a cover-glass. They may be easily cleaned off with turpentine or xylene and used over and over.
§ 317. Coating the lantern-slide glass with varnish. — One of
the best varnishes for this purpose is composed of 5% dry Canada balsam or gum dammar in xylene or in turpentine; or 10% natural Canada balsam in xylene or toluene. Or one can take some good, varnish, especially Valspar, one part and xylene, toluene, gasoline or turpentine nine parts. All of these thin solutions should be allowed to stand until they are clear, and only the clear part used. If one is in haste it is possible to filter the thin varnish through filter paper.
For coating the glass, the best way is to hold the clean glass flat by grasping the edges with the thumb and fingers. Then varnish is poured on, and the glass tilted slightly until the whole surface is covered. The excess is poured off one corner back into the bottle. Then the glass is stood on edge to dry. In a warm dry room 15-20 minutes will suffice for varnish in xylene or toluene. If turpentine is used it may require half a day or more. When the varnish is once dry the glass can be used at any time.
As it is not easy to tell which side has been varnished, a slight mark in one corner of the varnished surface with a glass pencil or pen will enable one to tell quickly and with certainty.
§ 318. Coating the lantern-slide glass with 10% gelatin. — For
this, some clear gelatin is made into a 10% solution in hot water,