We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Types of Visual Aids and Their Uses 83
to this projector at a further cost of $38.50; or a micro-slide attachment can be added to the objective lens of the projector at a cost of $20. Thus it is possible to equip one projector for the use of glass slides, filmslides, and micro-slides. In most cases, however, it is not advisable to attach so many different things to the one projection unit. The use of the equipment is thus limited to one room at any one recitation period. If separate instruments were available, it would be possible to use them in two or more different parts of the building. Furthermore, the cost of a separate filmslide projector is approximately the same as the cost of a filmslide attachment for the glass-slide projector.
A similar combination of equipment can be assembled by adding these various attachments to a standard opaque projector. The opaque projector costs $100 and a glass-slide attachment costs $35 more. A filmslide attachment can be added for $38.50 and the other attachments as mentioned above at the prices quoted. The same objection is raised to this elaborate combination; it is less portable and is restricted to the use of one type of service at one time. It may be desirable to have one or two special attachments for the glass-slide lantern, and the extent to which the equipment is to be used in one room would be the determining factor. If several teachers in a school building are planning to use projected visual aids of various kinds, there should be as many different projection units as possible rather than a combination outfit.
Principles of Projection. The above discussion has mentioned some of the principles which should be applied in the projection of pictures. There are certain factors which determine the size of the picture: the distance from the screen, and the focal length of the lens. There are other factors which determine the quality of projection: the strength of the illuminant, the quality of the equipment itself, and the interference of light. Daylight projection has been mentioned earlier in this discussion. Perhaps it is well to mention it again inasmuch as there are so many incorrect ideas concerning daylight projection. As a matter of fact, there is no such thing as a "daylight" projector. The projector of today is many times as efficient as those which were in common use fifteen to twenty years ago. It is true, also, that these more efficient instruments will give reasonably good results in situations where it would be impossible to secure even passably good pictures with the old types of projectors. This does not mean that daylight projection, or anything approaching it, is entirely satisfactory. It