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Condensing Lens Fire Shutter >
Aperture Plata Objective
Screen
on the screen can, then, be controlled within limits by the current which is forced through the source. The condensers in all arc lamp systems must be located at a considerable distance from the source, chiefly because of the intense heat of the arc, and manufacturing considerations limit the sizes of condensers to the point where, as you will note, the angle or zone subtended by the condenser with respect to the arc is small. With direct-current arcs this is not so great a handicap as with alternating-current arcs, for the direct-current arc throws the greater portion of the light it generates forward. The objectives used with arc lamp equipment are made small in diameter with respect to their focal length in order that irregularities in the arc will not be annoyingly apparent on the screen. There is a possibility of increasing the screen intensity with arc lamp apparatus by increasing the aperture of the objective lens, but under present conditions such an increase would be likely to be obtained at a sacrifice of steadiness in the picture. There seems to be a possibility of increasing the intensity produced by direct-current machines by more careful design of the shutter and possibly by rotating the shutter at higher speeds. It is doubtful if the shutters of alternating-current machines can be much improved because the cooling of the carbons is so rapid that a stroboscopic effect is likely to be encountered. This effect is, of course, marked at present where 25-cycle current must be used.
OpricAL Systcm U31M6 Mat&a clamp
You will note that with the incandescent lamp system, Fig. 6, a mirror can be used to redirect into useful directions the light which the lamp gives off in directions away from the condenser. The use of this mirror increases the illumination on the screen by approximately 75%. The lamp is so designed that relatively little light is thrown off to the sides, that is, a relatively large part of the light either strikes the condenser directly or indirectly