HOW DOES THE SKY GIVE LIGHT? 239 



Suggested Projects. 



1. MAKE A TOY PERISCOPE. 



2. TO USE A LARGE GLASS AS A MIRROR FOR " STAGE TRICKS." 



See Book of Magic, HOPKINS. 



Light and mankind. Have you ever, while looking 

 out of the window, put your hands over both eyes, and 

 then tried to imagine what life would be without light? 

 What a dismal place the earth would be ! We have come 

 to depend very 

 much upon the sun 

 and its radiant en- 

 ergy, which travels 

 to us across an 

 ether space just as 

 heat does, and our 



homes have been The sun is the source of all energy. 



planned so that we can make use of all sources of light, 

 both natural and artificial. Much of our light comes 

 from the sky rather than directly from the sun. The area 

 of the sky visible from our windows is very important, 

 because it is from this that we get most of our light. The 

 sun at midday is so bright we can look at it only with 

 strongly smoked glasses, and we all enjoy the long twilight 

 of summer following the light of day, when no sun is 

 visible. 



How does the sky give light ? This seems a simple 

 question, but in order to understand how light comes to 

 us, we must first learn some very important properties of 

 light. If we place a lighted candle or lamp on the table, 

 we can all see it. If we hold a piece of window glass in 

 front of it, the glass does not in any way obstruct the light. 

 We call such an object transparent. On the other hand, 



