428 THE OUTGO OF ENERGY 



a lens with aperture greater than 10 are not 

 refracted to the principal focus but cross the 

 principal axis between the principal focus and 

 the lens. The caustic surface formed by the 

 intersection of these peripheral rays may readily 

 be shown with any lens or cylinder of small 

 radius of curvature. 



2. Place the diaphragm with 2 mm. aperture 

 in front of the condenser. Throw parallel rays 

 into the optical box. Set in the box near the 

 window the cylindrical bottle of clear glass filled 

 with water. The bottle will serve as a powerful 

 refracting cylinder. 



The circular pencil of parallel rays will be 

 brought to a focus in a vertical line (compare 

 page 422). The outer rays of the pencil pass 

 through the outer portion of the cylinder, arid 

 are therefore more strongly refracted than those 

 near the optical axis. Each refracted ray inter- 

 sects the refracted rays nearer than itself to the 

 principal axis. These intersections form two 

 curved surfaces extending from the principal 

 focus in this case a vertical line towards 

 the cylinder. On regarding these surfaces from 

 above, their curvature will be apparent. 



3. Kernove the projecting lenses; place the 

 ground glass plate and the diaphragm with 2 mm. 

 aperture in front of the condenser. Let the rays 



