228 



GENERAL SCIENCE 



faces or one curved and one plane surface. The curved 

 surfaces are usually spherical and may have any of the 

 forms shown in Figure 202. There are two general classes 



FIG. 202. Lenses of Different Forms. 



of lenses, convex lenses which are thicker at the middle 

 than at the edge and concave lenses which are thicker at 

 the edge. 



Experiment 66. Hold a double convex lens in the sun's rays. 

 Scatter some crayon dust under the lens and notice how the re- 

 fracted rays cortverge to one point. This point is called the focus 

 or " fireplace." Hold a piece of tissue paper at this point. What 

 is the effect? View some print through this lens. Does the lens 

 magnify or not? 



Experiment 67. Draw the blinds on all the windows in the 

 room except one. Hold a convex lens near the wall opposite the 

 window, and then move it until a clear image of the window is 

 thrown on the wall. The distance from the lens to the wall is 

 called the focal length of the lens. 



When rays of light pass 

 through a lens, they are bent 

 toward the thickest part of 

 the lens. 



Uses of the Lens. The 

 *> simple microscope is nothing 

 FIG. 203. A Convex Lens Used as more than a convex lens as 



a Magnifying Glass. . .~. ^f\f\ * 



shown in Figure 203. An 



object viewed through such a lens appears in its natural 

 position, but larger. Spectacles are often but a pair of 

 convex lenses of small magnifying power. 



