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CH. VI] CONDENSER FOR SUNLIGHT 161
CONDENSER FOR SUNLIGHT
§ 256. As sunlight is composed of practically parallel rays, the condenser consists of a single plano-convex lens with the convexity receiving the light (fig. 74); or one may use an achromatic combination (fig.324).
The condition is practically like the ordinary condenser after the light has been rendered parallel by the first element of the condenser (fig. 3). Having parallel rays to start with, only the second element of the condenser is needed.
§ 257. Increasing the illumination. — The greatest difference between the use of sunlight and the arc light for projection appears when one wishes to increase the illumination. With the arc lamp one simply uses more current, and this increases the candle-power and makes the screen image more brilliant. With the same size condenser and picture the illumination of the screen with the arc light is directly proportional to the illumination of the condenser face.
With sunlight, the illumination of the condenser face is a constant quantity except for haze, etc. As all the light which strikes the screen must pass through the condenser, the screen illumination can be increased with sunlight only by using a condenser of larger diameter and correspondingly greater focal length. And for this one must have heliostat mirrors of sufficient size to fill the condenser with light.
§ 258. The water-cell with sunlight. — This light is accompanied by so much radiant heat that it is desirable to use a watercell with the apparatus, and thus reduce the liability of over-heating lantern slides or other specimens used for projection (see § 848 for the discussion of the need of a water-cell) .
CONDUCT OF AN EXHIBITION WITH SUNLIGHT
§ 259. The general principles given in Ch. I, § 21-41 are applicable.
§ 260. Lighting of the room. — Sunlight is sufficiently powerful so that the room used need not be very dark for showing lantern