338 



GENERAL SCIENCE 



focus no image will be formed. The rays of light will spread farther 

 and farther apart. 



FIG. 215. The lens of a lighthouse. Notice how each part of the lens is so 

 constructed as to cause the rays of light to pass out in parallel beams. 



Condensers in Stereopticons. Lenses called condensers are 

 used in stereopticons to produce parallel rays. A transparent pic- 



E 



o 



FIG. 216. How the rays of light pass through the lens of a stereopticon. 



ture 2| by 3 inches, called a slide, is placed as near the condenser 

 as possible, so as to be equally illuminated by the parallel rays coming 



