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THE LIGHT S0URC1 375
is found where there is both maximum illumination and evenness of light distribution upon the screen. When you get that lock the mirror into place. The position of
the mirror should be checked at reasonable intervals to make sure that it has not been disturbed.
(98) In the matter of lubrication the projectionist should follow trie recommendation of his lamp manufacturer. Not all of them use the same materials for bear ings of moving parts. Some demand that no lubrication be applied to any part of their lamp. Other manufacturers suggest lubricating certain parts only, but not all agree on wnat lubricant, is best.
For lubrication of parts working in high temperature we firmly believe that a mixture of pure powdered graphite and kerosene is best. Some lamp manufacturers condemn its use, claiming it clogs up the bearings, but this, we believe, is due wholly to the use of impure graphite or graphite mixed with grease. Graphitegrease mixtures must not be used at all. We suggest that the lam]) manufacturer's recommendation be followed implicitly and in the event trouble arises from the use of an improper lubricant, the manufacturer will have to bear all responsibility.
Old Style D.C. Arc
(99) Since this type of light source is now seldom used for theatre projection (because of its poor efficiency it should not be used at all), it is neither necessary nor desirable to deal with it extensively. In these lamps cored carbons of proper diameter should be used, neither over nor under loaded, with the negative tip adjusted so that the crater of the upper carbon. will face the collector lens.
New High Intensity Low Amperage Carbon Arc
(100) Prior to the development of the two new high intensity light sources (described later), there existed a wide gap between the highest screen illumination practicable with low intensity reflecting arcs operating on direct current at 16 to 42 amperes at the arc and the lowest