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814
ARTHUR C. HARDY
[J. S. M. P. E
(1) The projection lens must remain the aperture stop of the entire system.
(2) The gate must remain the field stop of the entire system.
These conditions do not determine the best design of the illuminating system. However, since the cost of operation of either *a tungsten
Gate
Screen
Condenser
Arc
• FIG. 3. A system in which the arc crater is focussed on the projection lens
lamp or an arc is approximately proportional to its size, it is more economical to satisfy the above conditions with as small a source as possible.
There are two illuminating systems that possess more than ordinary interest. These are shown in Figs. 3 and 4, but we will consider the system of Fig. 3 first. The arc is here focussed on the projection lens by a condenser located at or very near the gate. It is obviously impossible with this arrangement for the arc to limit the field or for the
Screen
Arc
FIG. 4. A system in which the arc crater is focussed on the gate.
condenser to limit the aperture. The condenser will not be the field stop of the system if it is larger than the gate and the arc will not be the aperture stop if its image fills the entire area of the projection lens. The magnification of the image of the arc should be as high as possible so that a small source can be used. This means that the condenser should have a short focal length, which requires that the source be placed very close to the condenser. The limit of efficiency is reached with this system when the focal length of the condenser is as