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118 RICHARDSON'S BLUEBOOK OF PROJECTION
mately half the light at one swoop. Deposits of dust and scum gathered from the air also cause loss of light (by reflection) in exact proportion to the amount and character of the deposit.
It is easy to see how important it is to keep all lenses scrupulously clean and well polished. (71) Loss of light is not the only evil effect of dirty lenses. Deposits of dust and dirt disperse or diffuse and misdirect a portion of the light rays and as a result the lighter shades in the screen image are made still lighter and the blacks become a dirty gray. The picture, instead of being "snappy" is dull.
Cleaning Lenses
(72) Oil on a lens surface seriously injures the definition (focus) of the screen image. Oil, once deposited, spreads around. (73) A slight finger mark, so faint it cannot be seen by looking through the lens, may blur definition. (74) Such marks usually may be seen by looking at the glass surface sidewise.
(75) There are many lens cleaning preparations on the market, but best of all is a soft cloth and a mixture of half clean water and half grain alcohol. Do not use denatured alcohol as it leaves a scum on the glass. Clean the lens when it is cold. Be careful not to touch the lens with your fingers after cleaning it.
(76) Lenses should be examined at least once each day. (77) Mirrors or condensing lenses should be cleaned each morning while they are cold.
(78) At regular intervals, lenses should be disassembled, the coating on the interior of their barrels examined and the lens surfaces cleaned. (79) The interior coating of the barrel is dull black to prevent reflection of light. (80) If it wears or peels off so that the metal shows, it should be recoated immediately, using coach painters' black thinned with turpentine. In reassembling lenses, each element must be replaced with its heaviest curvature toward the screen end of the lens.
(81) One lens manufacturer tapers the various lens edges so that they cannot be replaced wrongly. Some