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Types of Visual Aids and Their Uses 67
plete array of opportunities for the use of projected still pictures. A further advantage of the glass slide is that a carefully constructed slide either of the homemade or of the commercial variety offers the maximum of brilliance in the projected image. Another great advantage of the projected slide is that the image remains absolutely steady, and it may be left on the screen for any desired length of time. In those cases where the slide is being used for testing purposes, it is possible to time accurately the period of projection. In this way it is possible to adapt the slide to research projects in which it is desirable to control the time element.
The greatest advantage of the glass slide over the majority of the other projected visual aids is that it may be used quite successfully in a room which has not been darkened thoroughly. There has been much said about daylight projection, and approximately 90 per cent of what has been said should be discounted as being enthusiastic misrepresentation. There is no projected picture which will compete successfully with the interference of sunlight or very direct daylight. There are all sorts of schemes which may be used to secure a reasonably clear picture under adverse conditions where the light may not be subdued, but it is usually desirable to darken the room to a reasonable extent. This is not necessary, of course, with the outline slides such as the outline maps of states, countries, or continents, nor is it necessary when using slides made from diagrams, typed on cellophane or other homemade slides with distinct outlines. If slides containing great varieties of shadings or colorings are used, however, it is not desirable to project them in a well-lighted room. The results are not the best, and in most cases it is but a small task to shut out interfering light. In those cases where it is practically impossible to shut out light interference, the glass slide will be found to be the most successful competitor with that light, and may be used under conditions which would practically eliminate the other forms of projected pictures.
Those who are considering visual instruction materials should give some attention to the availability of the type selected. An unusual type of visual aid, even though it may be very satisfactory, may prove to be an unwise investment due to its unavailability in various sections of the country. The glass slide is almost universal in its availability, and the majority of the service bureaus which have visual aids available for loan have very complete assortments of well-organized glassslide sets.
Another convenience of the glass slide is that it may be made from almost any type of drawing or photographic negative. Simple camera