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Types of Visual Aids and Their Uses 51
of the older school buildings, the matter of darkening brings into consideration the problem of ventilation. The newer buildings, with centralized and controlled ventilation, have eliminated this problem.
Another limitation, relatively unimportant, is that the opaque projector is more cumbersome to take from place to place than is the glass slide or filmslide projector. Although this is true, the weight of the newer types of opaque projectors for classroom service has been reduced to the point where any pupil in the upper grades or any teacher can take the projector from place to place with ease.
Some of the older types of opaque projectors damaged some of the materials being projected, particularly if the pictures were highly colored. The leading manufacturers of these projectors* have solved that difficulty by providing suitable ventilation for the smaller projectors, augmented by cooling fans in the more powerfully illuminated units.
The Stereograph and Stereoscope
The stereograph is a picture which produces the impression of the third dimension — depth. The pictures on the stereograph are taken with a two-lens camera, approximating the views on the retinas of our eyes as we look at any object or scene in its natural setting. Although it is an artificial creation of the third dimensional effect, the stereograph gives us the nearest pictorial approach to the object itself in its natural setting. The observer really sees one picture with the left eye and another with the right. The right eye sees more of the right side of the object. and the left more of the left side. The sense organs put the two pictures together and we see the whole object, thus giving the impressions of solidity and relief.
"The stereoscope itself is an optical instrument with a similar pair of lenses separated by a small wooden or metal partition to keep the right eye from seeing the left view and the left eye from seeing the right view. These lenses are arranged within a hood which fits over the eyes and tends to shut out the light and other possible distractions. When the stereograph is seen through this binocular instrument an impression of depth, or third dimension, is received. This gives charm and educational value to the picture, as it creates an illusion of reality and seems to transport one actually into the pictured situation. 'We see something with a second eye and the mind feels its way into the very depth of the picture, around the object, and gets an idea of its solidity,' said Oliver Wendell Holmes, who perfected this remarkable device.
*For information concerning opaque projectors, write to the companies mentioned on page 217.