424 THE OUTGO OF ENERGY 



form upon the screen a horizontal line with 

 blurred ends. 



Place the vertical slit in front of the con- 

 denser. 



A broad, faint, horizontal line with blurred 

 ends will be observed. Draw a diagram illus- 

 trating the formation of this image. 



Turn the cylinder, so that the curvature shall 

 lie in the horizontal meridian. 



The horizontal rays are at once united in a 

 narrow sharply defined vertical line with blurred 

 ends. 



EEFKACTION THROUGH COMBINED CONVEX AND 

 CYLINDRICAL LENSES 



Thus far segments of perfect spheres or cylin- 

 ders have been considered separately. In the 

 eye both the cornea and the lens are frequently 

 more convex in one meridian than in another. 

 Such surfaces can be obtained by combining a 

 convex with a cylindrical lens. 



1. Place the diaphragm of 2 min. aperture in 

 front of the condenser. Throw parallel rays 

 into the box. Place the convex lens in the axis 

 of the pencil next the window. Receive the 

 image of the illuminated aperture upon a screen 

 placed at the principal focus. The image will be 

 a well-defined circle. Place the cylindrical lens 



