244 SCIENCE OF COMMON THINGS. 



How a multitude of persons see the same object. Shadows. 



that the eclipse takes place eight minutes earlier than the calculated 

 period when the earth is in the nearest point of its orbit, as at T, and 

 eight minutes later when she is in the opposite part of her orbit, as at 7", 

 the difference in the distance of these two points from Jubiter being 

 190,000,000, of miles. 



1638 Why can a thousand persons see the same object at Hie same 

 time? 



Because it throws off from its surface an infinite 

 number of rays in all directions ; and one person sees 

 one portion of these rays, and another person another. 



16J39 Why can we not see the stars in the day-time f 



Because the light of the sun is so powerful that it 

 eclipses the feeble light of the stars ; in consequence 

 of whicli they are invisible by day. 



1Q3O In what manner is light propagated f 



In right lines from every luminous point, every such 

 line being called a ray of light. 



1631 Wliat do we mean by a pencil of 

 light? 



A collection of radiating lines 

 or rays, as seen in Fig. 51. 



163S What is darkness? 



The absence of light. 



1633 What is a shadow? 



A shadow is the name given to the comparative 

 darkness of places or objects, which are prevented by 

 intervening things from receiving the direct rays of 

 some luminous body shining on the objects around. 



1634 Why cannot we see through a crooked tube as well as through a 

 straight one ? 



Because light moves only in straight lines. 



1635 What is the philosophy of taking aim with a gun or arrow ? 



In taking aim with a gun or arrow, we proceed upon 

 the supposition that light moves in straight lines, and 

 try to make the projectile go to the desired object as 

 nearly as possible by the path along which the lig/it 

 comes from the object to the eye. 



1636 Why does a carpenter look along the edge of a plank to see if it 

 is straight ? 



