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90 The Audio-Visual Handbook
Because of these and possibly other advantages, the increase in the use of filmslides in less than two decades has placed in the schools of the United States more pictures in this form than there are reported
300-watt Miniature Slide Projector
Photo Courtesy Society for Visual Education, Inc.
to be glass slides after more than a half century of usage. There is more new educational visual material available in filmslide form at present than in any other. It is interesting to note that in recent years the demand on the United States Department of Agriculture for stereopticon pictures is now solely for filmslides, whereas as late as 1932 thousands of glass slides were requistioned annually for loan.
Other Government agencies are using filmstrips for training purposes and have found these convenient small pictures to be unusually satisfactory.
Limitations. The principal earlier limitation of filmslides was chiefly the limitation of projection. It was difficult to obtain sufficient brilliance of projection without danger of damage to the film. The development and use of efficient heat-ray filters has dissipated this danger, without reducing materially the projection efficiency. The temperature at the aperture gate, which is important in the projection of all types of still pictures, has been reduced to the point where it is no longer a problem. This makes it possible to use 200 and 300-watt concentrated filament lamps for projection. With these, it is possible to secure projection results which are entirely satisfactory under all ordinary circumstances.