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212 RICHARDSON'S BLUEBOOK OF PROJECTION
carbon when the arc is struck, but it is also, to a considerable extent, a protection against the effects of sudden temperature changes. It has saved many a lens from cracking.
(16) The high intensity dowser blade should be of substantial thickness to withstand the extremely hot light source. A thin blade is useless against the heavy and (at high amperage) long tail flame of the light source. It is either warped or wholly destroyed.
(17) Some lamphouses are equipped with two dowsers, one located between the light source and the condenser and the other in the lamphouse cone. It is presumed to be a double protection for the condenser against sudden temperature changes.
(18) Dowsers for the low intensity reflector type and the mazda type light sources may be located in the lamphouse cone.
(19) The dowser must be kept closed when striking a straight high intensity arc, and kept closed until the arc is burning normally. Never open an inside dowser after pulling the light source switch until the lens has had time to cool off somewhat.
Inspection
(20) The condenser lenses, particularly the collector lens of a two-lens condenser, or the mirror collectorconverger, are exposed to deposits deriving from the gases generated by the arc or carbon dust. Other particles in the air also collect upon them. Therefore they should be inspected and thoroughly cleaned each day, either by washing with soap and water, followed by a thorough rinsing and polishing, or by wiping with a cloth wet with an equal mixture of grain alcohol and water, followed by thorough polishing. Where a high amperage arc is used the collector lens or mirror should be wiped off after each show. (21) The whole lamphouse should be inspected and thoroughly cleaned once a week at least, but more often in theatres of continuous performance. Frequency of cleaning is dependent to a considerable extent upon the kind and power of the light