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LENSES AND MIRRORS 131
renewed so often, but current bills will be bigger or as an alternative screen illumination will be decreased.
The Collector
We now arrive at the collector, which may he either a glass lens or a curved mirror. (125) If it is a lens ( a piano convex condenser) it should be a 6.5-inch focal length lens, (126) with the focal length of the converging lens sufficient to provide the necessary spot diameter at the aperture and a minimum distance from the light source to the collector. This applies only to the piano convex 2-lens condenser. (127) The lenses arc placed with their curved surfaces together for two reason-. First, were the collector lens placed with its curved surface toward the light source, the reflection from its outer zones would be very high — in fact, if the distance from the light source to the lens is short, the reflection at its outer zones would include almost all the light incident on these zones. Second, with its curved surfaces together, opticians tell us there is less spherical aberration tli an when mounted the other way.
Spacing Condenser Lenses
(128) Condenser lenses should be as close together as possible without actual physical contact. Contact would transfer heat from the very hot collector to a single spot on the relatively cool converging lens. (129) As Fig. 43 shows, the collector does not send forward a parallel, hut a diverging beam. The actual separation need he no more than 1/64 inch but it must be enough to prevent actual physical contact.
The Converging Lens
With a 2-lens plano-convex condenser, use a 6.S inch focal length collector in conjunction with a converging lens of sufficient focal length to give the required spol diameter at the desired distance. (130) The spot diameter will be dependent upon the focal length of the combination, when the distance from the converging lens to aperture has been fixed.