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VISUAL-AUDIO MEDIA OF A CLASSROOM 439
An important advantage of the slidefilm over transparencies quite apart from its low cost is that the scenes of the film are certain to be projected in correct sequence.
To reduce the number of machines needed, a number of manufacturers have sought to combine two or more of these projection functions in a single mechanism. The most common combination is that of the 35-mm slidefilm and the 2 x 2 in. slide. Although this compromise need not involve a serious loss of optical efficiency because the image areas to be projected are not greatly different, the compromises that must be made if design simplicity is to be maintained become serious when an attempt is made to include the larger 3 1/4 x 4 in. transparency. The compromises become very serious when the 6x6 in. opaque material function is also included.
When there is a material difference in the size of the transparency or opaque material to be projected, not only the projection objective lens but also the condenser lens must be changed in focal length if good light efficiency is to be achieved with all sizes. In the optical system, the light source is imaged near the aperture where the transparency or opaque material is located; the condensing lens system should adequately and uniformly illuminate the material without serious waste of illumination beyond its boundaries. The area of the light source is fixed, and a change in the focal length of the condenser system is required, just as is a change in focal length of the projection objective lens.
Admittedly, it is undesirable for a classroom to "bristle" with projection machines and audio aids when only one facility can be used at a time. Thus, as a practical matter, it is usually justified for the sake of operational convenience and simplicity to sacrifice illumination efficiency to some degree in a combination machine if the screen brightnesses of the facilities so furnished are not too widely different from one another.
The real problem, however, goes deeper. There is still much room for improvement not only in providing apparatus integrated with the teaching program (as well as the integration of the apparatus components with each other), but also there is a far more serious need for the improvement of teaching techniques by providing visual-audio materials using the most effective facilities integrated as a part of the teaching method and fitted accurately to the specific teaching need.
As one example, it is well known that the amount of "dwell" time needed for the contemplation of an important point varies from one audience group to another. One possible arrangement that can take this into