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CH. VIII] PREPARATION OF LANTERN SLIDES 217
mark on it so that the operator can put it into the lantern correctly without closely inspecting each slide.
Unfortunately there is no general system of marking slides. The method recommended by the British Photographic Club (Bayley, p. 78) is to put two white spots on the upper edge of the slide (fig. 113). Two spots are necessary for the square slides, but for oblong slides one "spot" or mark is sufficient (fig. 112).
In America it is common to have the mark or spot on the lower left hand corner of the slide (§ 112), then when the slides are in a pile for inserting in the lantern the spot will be turned upward (fig. 8) as it must be to give an erect screen image. In the British method of "spotting" the slides would have the spots on the lower edge when piled up ready for insertion in the lantern.
§ 336. Coloring lantern slides. — Photographic lantern slides have been colored from their first production. To do this in the best manner possible requires considerable practise and natural artistic ability, but any one can color lantern slides sufficiently well to add to clearness in teaching — for example, veins blue, arteries red, etc. All that is needed is a small artist's brush and some of the desired color.
Transparent colors in sets are on the market (see Appendix), or one can employ the aqueous stains used in histology. It takes some experience to get the right dilution of the color and to put it on neatly with the brush. The slide should be held over some white paper in a light place so that it is possible to see exactly what is being done. The frame for holding slides is a convenience (fig. 113).
If one wishes to become expert it will be a great help to study the works of reference given at the head of this chapter, for they give many valuable hints.
One very important thing for the beginner to do is to test every slide that is colored in the lantern to make sure that the colors look right in the screen image. Sometimes a slide that looks well to the naked eye in daylight will not look well when projected on the screen. It is, of course, the screen image that must be satisfactory.
The early lantern slides were mostly colored with transparent oil colors, and then when entirely dry, the slide was mounted in Canada balsam, and a cover-glass put on exactly as microscopic