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MANAGERS AND PROJECTIONISTS 305
that the condenser be moved up and down l/64th of an inch more than the projection lens. This would set up a condition such as is shown in Fig. 83 and the effect on the picture on the screen would increase in proportion as the distance from the lens to the screen increases. In Fig. 83 A is the light source and B the projection lens. If A be moved down l/64th of an inch, B remaining stationary you will readily see that the movement of the light beam at a point say 100 feet away will be considerable. The dotted line illustrates the result.
Modern practice is to provide a solid foundation upon which is placed not less than six inches of a rich concrete, well tamped down. On this is placed a top dressing of
Figure 83.
cement from .5 of an inch to one inch thick, essentially of the same composition as is used for side walks.
WARNING. — An enormous amount of damage is done both to machinery and to film by improper mixing of the top dressing of projection room floors, or by the use of poor cements. If the top dressing be not properly mixed, or if it be of poor material, it will constantly wear off, and the resultant dust is a very fine abrasive powder. It gets into the bearings of projectors, motors and generators and wears them out very rapidly. It gets on the film and is one of the greatest rain producers known. We have seen many projection rooms in which an improper top floor dressing was shortening the life of the machinery enormously, as well as doing immense damage to the films used therein.
FLOOR DRESSING.— Where such a floor exists it is possible to apply a preparation which will stop the trouble, or the floor may be painted with a good floor paint. Exhibitors will do well to remember that a projection room cement floor which slowly disintegrates into dust is about as great a damage producer as they can have in their theatre, and one great trouble with this particular thing is that the damage done is so nearly imperceptible that neither the exhibitor or the projectionist realizes how serious it really is.
One of the best possible coverings for a projection room floor is a heavy matting made of a cork composition, such