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138 RICHARDSON'S BLUEBOOK OF PROJECTION
(149) Silver will tarnish when exposed to air. To overcome this tendency it is placed upon the polished back of a curved glass mirror and (150) upon it is laid a coat of a heat resisting, non-porous covering called "mirror backing." The glass itself, of course, protects the silver on the other side.
In mirror reflectors of the parabolic or elliptical type (151) the light source must be located at a certain fixed distance from the mirror surface, and the mirror surface in its turn a certain fixed distance from the projector aperture. Any departure from these distances, particularly of the distance from the light source to the mirror, will mean loss in efficiency. (152) The distance from the light source to the mirror vertex is called the "mirror focus." (153) The distance from mirror vertex to the aperture is called the "mirror working distance."
(154) For best results mirrors must be operated at exactly the correct focus and working distance. (155) Distances are not the same for mirrors of different focal lengths. (156) An error of less than one millimeter in mirror focus distance is a very serious matter. It is therefore well to proceed with great caution in making changes in the adjustments of the mirror, especially when changing mirror focal lengths. Where slight alterations may produce such serious results it is a much better plan to follow the manufacturers recommendations and instructions very carefully, varying from them only when you are positively certain that results will be improved. You can secure instruction books freely by writing to the manufacturers.
(157) For low intensity lamps, it is advisable to use mirrors made of high grade optical glass which will not crack readily from heat if correct distances are maintained. When using the reflector type high intensity lamps, however, the mirror must be of high grade, heat resisting glass (pyrex, for example), if excessive breakage is to be avoided.
Care of Mirrors
(158) Mirrors cannot function efficiently unless the