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MANAGERS AND PROJECTIONISTS 169
collector lens and a bi-convex (convex on both sides) converging lens.
The plano-convex is the one most in use. It is used with the convex sides of both lenses next each other, which has the advantage of producing somewhat less spherical aberration, and a somewhat less loss by reflection from the surfaces of the lenses. Aside from this there is no difference in results when the flat sides of the lenses are placed together.
The Cinephor parabolic condenser (see page 212a) is a condenser which is in large measure corrected for spherical aberration. It has a decided advantage when the projection lens working distance is long; also where there is a heavy projection angle, since it permits of using a projection lens of smaller free diameter, and as lens diameter is reduced, other things being equal, depth of focus is increased, which means that the lens will have increased power to focus things at different distances from it.
The advantage of this is understood when we consider that where there is a heavy angle of projection the top of the screen is much nearer the lens than is its bottom.
Under modern conditions we may discard everything except the plano-convex and the Cinephor parabolic condenser for arc light projection — except, of course, where the reflector type arc lamp is used. There is no longer need for the meniscus bi-convex condenser.
SPACING OF CONDENSER LENSES.—
No matter what your plano-convex condenser combination may be, the lenses should always be spaced so that the nearest surfaces are as close together as you can get them without actual contact — say l/16th of an inch apart if you are able to get them that close. Optically the best possible position would be with the surfaces actually in contact, but this would communicate heat from the collector lens to the converging lens by Figure 39*
mechanical means, which would be highly objectionable. The lenses should be just sufficiently separated to avoid this. Lens tables are calculated on the equivalent focus of con