CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 



139 



233. Heliostat. From the rotation of the earth on its axis 

 from west to east the sun seems to move over the face of the sky 



\ 



FIG. 74. MAGIC LANTERN WITH SUNLIGHT. 



5 Sunlight. 



Mirror The plane mirror serving to direct the sunlight horizontally into 

 the condenser. 



Condenser The single plano-convex lens serving to converge the parallel 

 beam of sunlight. (Compare the second element of the condenser in fig. 2). 



Ls Lantern slide. 



Objective The projection objective for projecting an image upon the white 

 screen. The projection objective and the condenser should be of approxi- 

 mately the same focus. 



c Center of the projection objective where the rays from the condenser 

 should cross. 



Axis Axis The principal optic axis of the condenser and of the projection 

 objective. 



Screen Image The image of the lantern slide upcn the white screen. 



The apparent diameter of the sun's disc is 32'36" in midwinter and 31' 32" in 

 midsummer, or it averages 32' 04" (Abbot, The Sun, p. 3; Ball's Astronomy, 

 p. 127). 



The apparent area of the sun's disc at a distance of one meter is determined 

 as follows: Its diameter is 32'o4" or .5343. One centimeter at a distance of 

 one meter subtends an angle of .573, hence at one meter the sun's disc would 



appear to have a diameter of i = .933 centimeters. The area of such a 



573 



circle is : ' \ IT = .684 square centimeters. 

 4 



. . .,.,,. Candle-power 



1 he intrinsic brilliancy is then, - 



Area 



288,000 



- = in round num- 



.684 



bcrs, 421,000 candle-power per square centimeter or 2,720,000 candle-power per 

 square inch. 



