28
JOURNAL OF CHEMICAL EDUCATION
ION H. MYER Universitg of Southern California,Los h g e l e s , California
WHILE preparing
projection slides for use in our chemistry classes,' a method of slide preparation was sought which would permit direct drawing or tracing onto a matte glass plate; yet would result in a completely transparent final slide. It is well known that pencil or India-ink tracings may be made directly on matte glass plates, hut the resulting slide is not really suitable, since transmission of light through the matte plate is poor--resulting in a dull dark background even with powerful lantern illumination. By a simple procedure, however, it is possible to make the matte plate completely and brilliantly transparent. The principle employed for this and similar purposes utilizes the fact that a rough matte glass surface becomes completely transparent if covered with a clear oil, lacquer, or glue which has the same refractive index as the glass plate. To protect the slide surface and to avoid any lens effects it was found desirable to cover the treated plate with a second clear glass plate, just as is done in binding photographic plates. The method is given in detail below. I t was also found convenient and quite economical to prepare ground-glass plates in our own laboratory, and the method of making these is also described briefly. Slides made by this process are completely transparent, permanent, protected from scratches and dust, very sturdy, and Inexpensive to prepare. Procedure for Preparing the Slides. The text, formula, or drawing to be projected is drawn on a matte glass plate, using India ink and a fine nib, or a drawing pencil of medium hardness. A little practice in the use of India ink on the matte glass surface may be required,
' KHAMSCH,N., "The use of lantern slides in the teaching of undergraduate chemistry," J. CHEM.EDUC.,28, 280 (1951).
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hut excellent results can easily he obtained. Errors may he easily corrected with a hard eraser, and if necessary the whole matte glass surface may be restored with a scouring powder. After the slide has been drawn, and well dried if ink has been used, it is made transparent by applying Permount s ~ l u t i o n . ~In the case of a 3 l / ~X 4-in. slide, 1ml. of the thickened sirupy solution is applied to the center of the matte plate and a clear slide-plate is pressed on. Any air bubbles are driven to the edge by even application of pressure. This cement has the same refractive index as the glass of which the plates are made and proved superior to Canada balsam, vinyl acetatechloride copolymer, etc. Matte glass slides can he obtained commerciallyJ or may be prepared as follows: A heavy glass plate is covered with a sheet of damp paper. On this non-slip flat base a standard projection slide is ground with the aid of another slide and 320-mesh Carhorundum. The Carhorundum is mixed with water and the two plates are ground together with a circular motion using even pressure. With some practice two slides can be made in this way within two to three minutes. After rinsing and drying, the plates are ready for use. Considerable experimentation showed that the 320-mesh Carboruudum gave optimum results. The completed slide-sandwich is dried for 24 hours a t 37'C., and bound in the usual manner.4
' This is a plastic cover-glass cement sold by Fisher Scientific Company, Catalogue No. So-P-15. It should be thickened by partial evaporation. a Available from Keystone View Company. 'We found Mystik Tape (a/+ in. wide) convenient.
