26 LIGHT 



There is another way in which sunlight helps us and 

 that is by killing disease germs. Much of our sickness 

 is caused by little plants, called bacteria, which are so 

 small that they cannot be seen. These plants grow best 

 in dark and damp places and for that reason we should 

 live in dry and sunny houses. If the sunlight which 

 comes in through many windows is too much for our 

 comfort we can shut it out, but if the windows are too 

 few in number there is no way of obtaining enough sun- 

 light. Windows are as cheap as the wall they take the 

 place of, and to have few windows is no economy. 



14. Light Travels in Straight Lines. 



What is meant by a straight line? When we say 

 "As straight as a string," do we mean a tightly pulled 

 string or a loose string? If carpenters wish to mark a 

 straight line on a building they rub some chalk upon a 

 string, stretch it tightly where they desire the line, and 

 then pulling the middle out a little, let it snap back upon 

 the wall. Some of the chalk leaves the string and makes 

 a straight line. Why is the line straight? Try making 

 straight lines with a chalk-line. 



Now that you know just what is meant by the word 

 "straight" you can see that light travels in straight lines. 

 If there is a .candle or a lamp in a room, can you go any- 

 where in the room so that you cannot see the light? How 

 does the light come to you from the candle or lamp, 

 straight or curved? 



Experiment 15. The Way Light Travels. 

 Apparatus: Slim stick three or four feet long, 

 string, two pieces of cardboard, candle. 



