AUDIO-VISUAL DEVICES 65

 Reference material such as charts, maps, graphs, flat pictures,
 posters, biological charts, and magazine articles serve as colorful,
 meaningful, informative aids. Vary the material to correlate
 with the topics under discussion.
 Models, exhibits, and specimens furnish the most concrete
 visual aids. Store these systematically in order to find them with
 minimum effort. Encourage students to collect material of this
 kind from local sources, and by exchange with students in other
 parts of the country. The secondary and elementary schools
 can also exchange specimens. Many manufacturers offer mate-
 rial free or at low cost. Magazines for science teachers are con-
 stantly listing these offers.
 Filmstrips, opaque materials, and slides are the most econom-
 ical and the least complicated of the aids that require a screen.
 An entire lesson may be woven around a few well selected slides
 or frames of filmstrip. The lesson can be carried on entirely by
 students. These aids may be used individually by students or by
 student groups to enrich their experiences.
 Slides both 2"x2" and 3 I"x4Y4" may be student-made as
 well as rented or purchased. The General Extension Division of
 the University of Florida has many of these. The opaque type
 projector may be used for thin objects, flat pictures, or pictures
 in books. Frequently a chart, outline, or map may be projected
 in outline on the board and filled in by students as the lesson
 develops. A project in which some of this material is prepared
 may prove to be a useful activity.
 Many local libraries have filmstrips for loan. The General
Extension Division of the University of Florida, the United
States Department of Agriculture and the United States Office
of Education distribute many free.
 The motion picture as an audio-visual aid. The unique char-
acteristics of the motion picture, such as animation, micro-
photography, time-lapse photography, and slow-motion, great-
ly enhance realism in science teaching. Since the motion picture,
both silent and sound, is an integral part of the curriculum, it
is best to have the films shown in the classroom itself. The re-
sponsibility of the teacher is to select the film, plan the proper