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208 PREPARATION OF LANTERN SLIDES ICn. VIII
§ 323. Smoked glass. — For some purposes nothing is better than smoked glass slides. On these one can write or draw with a sharp point either before or during the exhibition. If one takes the precaution to commence writing on the lower edge of the slide and on the face looking toward the condenser the writing or diagram will appear right side up on the screen (see § 3 5 for proper position of lantern slides in the holder) .
Smoked slides must be handled carefully or the surface will be spoiled.
§ 324. Thin sheets of mica or of gelatin. — On a sheet of mica, of gelatin or of non-inflammable cellulose one can write or draw with a pen or brush, using any colored ink. India ink is best for outlines and for written words, letters, or numerals.
As these sheets are very thin it is best to put a slide made upon one of them between two glasses, so that the sheet will be held flat and be protected. (For other methods of hand-made slides see Dolbcar, pp. 29-32).
PHOTOGRAPHIC LANTERN SLIDES
§ 325. Nearly all of the lantern slides now used arc made wholly or in part by photography.
Negative. — First, there is made a negative of the object to be represented in the lantern slide. This negative may be on any size of plate, but the picture should be, if convenient, of the proper size for a lantern slide. That is, its outside dimensions must not exceed 75 x 70 mm. (3 x 2.8 in.).
This negative should be very sharp and free from defects. Any lack of sharpness or any defects will come out with distressing prominence when the picture is magnified by the lantern. One must then use a good objective in making the picture, or if the objective is not particularly good a very small diaphragm is used. If it is desired that print shall be read easily by all in the room, the lantern slide should not have the letters smaller than six point type (see fig. 216 for sizes of type).