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124 The AudioVisual Handbook
5. If the picture is to be used extensively, have a "dupe" negative prepared from which additional prints may be made, and save the original. If the film is not to be used more than forty or fifty times, the duplicate print will not be necessary.
35 mm. Silent Motion Pictures
Very little attention has been given to 35 mm. silent motion pictures in this book inasmuch as films of that type are being used less and less among schools. The 16 mm. motion-picture films and equipment have replaced the 35 mm. silent films in the majority of the schools, and there are few instances in which 35 mm. silent subjects are being produced. Those who may be interested in using 35 mm. motionpicture service for any special purpose will be able to secure used films and equipment at very reasonable rates, but will find that the transportation charges on 35 mm. films are extremely high in comparison with the cost of shipping 16 mm. silent subjects. Furthermore, the quantity of good 35 mm. silent educational subjects is so limited that the average school will not be able to get what it wants on that size and type of film.
Screens for School Use
Satisfactory projection of pictures of all types requires, first of all, a screen with a reflecting surface suitable for the situation. There are many different types of screens available with trade names of every description. There is a rather general misconception of screens, as there has been of projectors, in that some screens are called "daylight"; there is no screen which will give entirely satisfactory service when outside light is interfering to any great degree. There are some screens which have higher reflective qualities than others, and the purpose of the following discussion will be to call attention to some of the advantages and limitations of these various types.
There are two general types of screens: (1) those which reflect the picture, and (2) those which transmit the picture. The first type is called the opaque screen and the second, the translucent. The reflecting or opaque screen is used most generally by schools and other educational organizations, although the translucent screen is used in some instances where it is advisable to have both the projector and the screen at the front of the classroom and it is necessary to compete with interfering light.