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264 RICHARDSON'S BLUEBOOK OF PROJECTION
cover ports but optical glass is relatively expensive and often good plate glass will serve as efficiently.
Optical glass should be used for the projector and stereopticon lens ports, but is not necessary for .either observation or spotlight ports. Good plate glass will not distort either vision or a spotlight beam. It probably has minute bubbles or stresses that might visibly affect the projector light beam but it would have no appreciably bad effect when used in stereopticon or effect projector lens ports.
Optical glass, therefore, is recommended for the projector lens ports and if preferred for the stereopticon and effect projector lens ports. For observation and spot lamp ports we recommend the use of a good grade of plate glass.
(86) Be careful that the projectionist's line of vision when he is standing beside the projector is not obstructed by light reflected from the surface of the cover glass over the ports. This can be avoided by setting the glass at an angle. The angle will vary according to the conditions of each projection room; therefore no fixed angle from the perpendicular can be named.
(87) The glass in all ports must be mounted to make the outer surfaces conveniently accessible for cleaning — an important item that must not be overlooked. This may be accomplished in one of several ways. The glass may be mounted in a metal frame and this, in turn, mounted in a second metal frame to be hung on the wall ; the frame holding the glass is hinged to the bottom of the wall frame, so that its top may be pulled back and the glass cleaned conveniently. Or the glasses may be set in grooves, except for one objectionable feature: the glass is subject to frequent breakage because it is handled so much.
Lens Ports
We do not favor covering the lens ports with glass unless tests prove it to be really necessary. (88) If the lens ports are reduced to approximately the dimensions of the light beam the openings will be so small that no