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 com- 

 bination (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 ele- 

 ment 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 con- 

 stant quantity except for haze, etc. As all the light which strikes 

 the screen must pass through the condenser, the screen illumina- 

 tion 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 accom- 

 panied by so much radiant heat that it is desirable to use a water- 

 cell 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 



