SCIENCE OF COMMON THINGS. 



253 



How telescopes assist the sight. 



Fig. 62. 



FiK.es. 



centre of the front of the eye. Noy, as the rays of light proceeding from 

 an object diverge, or spread out, the number which will enter the eye will 

 be limited by the size of the pupil. At a great distance from an object, 

 as will be seen in Fig. 62, few rays will enter the eye ; but if, as in Fig. 

 63, we place before the eye a piece of glass, called a lens, so constructed 

 as to collect all the diverging rays together, the light will be concentrated 

 at one point, and in sufficient quantity to enable us to see distinctly. 



1607 Why do telescopes enable us to see objects invisible to the naked 

 eyef 



Because they gather together more luminous rays 

 from obscure objects than the eye can, and form a bright 

 image of them m the tube of the telescope, where by 

 means of lenses they are magnified. 



1608 When a ship (out at sea) is approaching the shore, why do we 

 see the small masts before we see the bulky hull ? 



Because the earth is round; and the curve of the sea 

 hides the hull from our eyes after the tall moists have 

 become visible. 



Fig 64. 



Here only that part of the ship above the line A C can be seen by the 

 gpectator, A ; the rest of the ship is hidden by the swell of the curve D E. 



The diminution of the size of a ship seen at sea, owing to the convexity 

 of the earth and the distance of the observer, is also illustrated in Fig. 65k, 



1669 What is meant by the rejraction of light? 



Light traverses a given transparent substance, such as 



