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218 RICHARDSON'S BLUEBOOK OF PROJECTION
less light by reflection than a clean one. It did not happen often, and the effect could not be reproduced by intentionally soiling the lens ; but it did happen once in a while. Investigation in recent years has proved that the effect could be reproduced accurately by carefully controlling the thickness (or rather, the thinness) of the layer of tarnish on glass-to-air surfaces.
(170) Lenses made today are intentionally "treated" or "surfaced" at the factory to reduce their reflection losses. Sub-microscopically thin layers or Alms are produced on the glass by chemical action. The thickness of these deposits is governed by careful control of the concentration of the chemicals used and of the length of time the glass is exposed to their action.
(171) Lenses purchased before the treating process came into use can be surface-treated either by the maker or by companies engaged in that business.
(172) Surface treatment, it is authoritatively stated, can reduce reflection losses in projection lenses by %rds. On this basis, where an untreated lens wasted SI out of every $3 spent for electricity to provide projection light, the same lens wastes only 33c out of S3 after it is "treated." And there is a corresponding gain in the sharpness of the image, and in the clearness of color pictures, through the reduction of ^rds in the amount of diffused light reaching the screen as a result of lens reflections.
(173) Treated lenses impose one important precaution on the projectionist. He must be very careful to clean them only as recommended by the manufacturer or by the company that applied the treatment. More than one type of treatment is known today, and the different coatings produced may be damaged by one or another method of cleaning. The projectionist should take the trouble to inquire into the correct method of cleaning his coated lenses, and adhere to it scrupulously. A projectionist, taking over a tiew assignment, whether temporarily or permanently, must be careful to inquire, before attempting to clean the lenses, whether or not they are treated, and if so, what cleaning method is recommended for the particular treatment applied.