OPTICS 



525 



Expt. 97. Refraction by Concave Lenses. (Apparatus: 10-diopter 

 concave lens, round diaphragm, screen.) Let parallel rays pass into 

 the eye, and through the 10 D. lens. The rays will be diverged. 



Expt. 98. Refraction by Cylindrical Lenses. (Apparatus: Cylindrical 

 lens (the thicker one, plane on one side and convex on the other), round 

 diaphragm, L-diaphragm, screen.) Make a pencil of parallel rays 

 pass into the eye and through the convex cylinder placed so that the 



FIG. 293 



Divergence of the rays in a beam of light is brought about by a concave lens. There 

 is no focus save the mathematical one at F. 



FIG. 294 





A cylinder in diagram to show the principle on which cylindrical spectacle-lenses are made: 

 a, 6, c, d is a concavo-convex "cylinder;" e, f, g, h, a plano-concave "cylinder." 



curvature is from side to side. Place the screen at the focus. Observe 

 that the cylindrical pencil of rays is no longer round, but that it forms 

 a vertical line on the screen. Rotate the lens 90. The line is now 

 horizontal. Move the screen back and forth on either side of the place 

 of clear focus and observe the evident shape of the mass of rays coming 

 from the lens. 



Repeat these procedures with the L-diaphragm in place of the round 

 diaphragm in the lantern. 



