346 BELL SYSTEM TECHNICAL JOURNAL 



the polarized light system the heat must be isolated from the color filters and 

 especially from the polarizing filter Pi, for it is extremely important that 

 polarizing screens should not be overheated. They deteriorate (lose their 

 polarizing property) rapidly above 60°C., and should not reach a tempera- 

 ture uncomfortable to the touch. Their deterioration by heat or otherwise 

 is not usually discernible except by examining their extinction quality with 

 the aid of a good piece of polarizing material (turned for extinction, they 

 should pass practically no light). Since the polarizing filters and the color 

 filters all absorb some light themselves it is important to ventilate these units, 

 as well as to isolate the mercury lamp heat with a separate housing. The 

 window HFi in the mercury lamp housing may be of one-eighth inch pyrex 

 glass, or better the heat filter IJF (specified later), and may be a moulded 

 filter (since there is no focusing required here). However, the window for 

 the projection lamp housing should be polished, either pyrex or heat filter 

 as above (since here the light is used in a focused beam). 



The polarizing filter Pi and P2 may be glass filters (polarizing film ce- 

 mented between glass plates by the manufacturer) or film filters held be- 

 tween sheets of one-eighth inch plate-glass, with taped edges. The latter 

 arrangement is less expensive, and the film is usually obtainable without 

 delay. Extra filters should be kept on hand. The two filters must be 

 mounted, relative to each other, in a crossed position (for extinction). Since 

 light entering the tank from Pi must be diffuse it is necessary to introduce a 

 ground glass surface. This is accomplished without adding an extra glass 

 plate by grinding one surface of the polarizing filter, the outer surface if a 

 cemented glass filter, an inner surface if using loose film between glass plates 

 (the inner surface may be used here to provide for protection from dirt). 

 In either case the ground surface must be on the lamp side of the filter or it 

 will depolarize the light. 



In the past a carbon arc has been used as the projection light source. 

 Such arcs are not handily turned on and off, nor adjusted, and are now dif- 

 ficult to obtain. An incandescent projection lamp overcomes these diffi- 

 culties. A 500 to 1000-watt lamp with double-plane filament structure 

 (filament in two planes, and staggered relative to each other, giving a solid 

 square of illumination) is ideal for this purpose. Such lamps operate at high 

 efficiency, are a concentrated source, have a short life, and generally require 

 forced ventilation. A recommended lamp is the Mazda, Clear Projection, 

 750W— 120V, T-12, C-13D Fil., Med. P.P. base. This lamp requires a 

 small blower for ventilation and when operated with a foot switch, only dur- 

 ing that part of the inspection that it is needed, gives a satisfactory replace- 

 ment schedule. 



It is important in the projection system to use large, short-focus condenser 

 lenses, and to focus the lamp image near the center of the tank. This allows 



