370 THE NATURE OF LIGHT. 



in straight lines. This is a fact of incalculable scien- 

 tific and otherwise practical importance. 



(O The familiar experiment of "taking sight" depends upon 

 this fact, for we see objects by the light which they send to the eye. 

 We cannot see around a corner or through a crooked tube. A beam 

 of light that enters a darkened room by a small aperture, marks an 

 illuminated course that is perfectly straight. 



(6.) This fact may be illustrated by providing two or three per- 

 forated screens and arranging them as shown in Fig. 272, so that 

 the holes and a candle flame shall be in the same straight line. 



FIG. 272. 



When the eye is placed in this line behind the screens, light passes 

 from the flame to the eye ; the flame is visible. A slight displace- 

 ment upward, downward or sidewise of the.eye, the flame or any 

 screen, cuts off the light and renders the flame invisible. 



(c.) Prepare a piece of wood, 1^ x 2^ x 18 inches, taking care that 

 the edges are square. Saw it into six pieces, each three inches long. 

 Prepare three pieces of wood, 3 x 4 x inches. Place three postal 

 cards one over the other on a board, and pierce them with a fine 

 awl or stout needle, \ inch from the end and \\ inch from either 

 side of the card. With a sharp knife pare off the rough edges of 

 the holes, and pass the needle through each hole to make the edges 

 smooth and even. Over the \ x 3 inch surface of one of the blocks 

 place the unperforated end of one of the postal cards, and over this 

 place one of the 3 x 4 inch pieces, so that their lower edges shall be 



