354 THE SUN'S GIFT OF LIGHT 



enough away so that the farther edge of the penny can be seen over 

 the edge of the pan. Have some one slowly fill the pan with water. 

 How is the visibility of the penny affected ? 

 (Figure 102.) 



Experiment 104. Fill a tall jar about 

 two thirds full of water. Place a glass rod 

 or stick in the jar. Does the rod appear 

 straight ? Pour two or three inches of kero- 

 sene on the top of the water. What effect 

 does this have on the appearance of the rod ? 

 FIGURE 102 Experiment 105. Hold an ordinary spec- 



tacle lens such as is used by an elderly 



person, or any convex lens, between the sun and a piece of paper. 

 Vary the distances of the lens from the paper. The heat and 

 light rays from the sun are bent so that they converge to a point. 

 Try the same experiment with a lens used by a short-sighted 

 person, or a concave lens. This lens does not have the same effect 

 as the convex lens. The rays are made to diverge. Why cannot 

 long-sighted and short-sighted persons use the same glasses ? 



In the experiment of the penny in the dish, the water 

 in some way bent the ray of light and made the penny 

 come into the line of sight when it could not be seen before 

 the water was there. The penny was apparently lifted up. 

 This illustrates why ponds and streams look shallower 

 than they really are. This experiment shows that when 

 light is passing from one medium to another it does not 

 always travel in the. same straight line. Certain media 

 offer more resistance to the passage of light than others and 

 are called denser media. It is this difference of resistance 

 which causes the bending of the ray. 



Suppose that a column of soldiers marching in company 

 front are passing though a corn field and come obliquely 

 upon a smooth open field. (Figure 103.) The men as they 

 come on to the open field are unencumbered by the corn- 



