253 SCIENCE OF COMMON THINGS. 



Why objects in the shade seem dark, t Telescopes. 



The rays A D and C D 

 appear to be no longer 

 than the ray B 1) ; but if 

 all the rays seem of the 

 same length, the part B 

 will not seem to be nearer 

 to us than A and C ' ; and therefore ABO will look like a flat or straight 

 line. The rays A D and C D are 240,000 miles long. The ray B D is 

 238,910 miles long. 



1603 An object in the shade is not so bright and apparent as an object 

 in the sun : why is it not f 



Because objects in the shade are seen by reflected 

 light reflected ; that is, the light is twice reflected ; 

 and, as the rays of light are always absorbed in some 

 measure by every substance on which they fall, and 

 also scattered by irregular reflections, therefore in the 

 two reflections much light is lost, and the object is seen 

 with less distinctness. 



Part of the rays are absorbed, and part are scattered in all directions 

 by irregular reflections ; so that rarely more than half are reflected, even 

 from the most polished metals. 



1664 Why is it light when the sky is covered with thick clouds ? 



Partially because the sun's light is transmitted 

 through the clouds, and partially on account of the 

 multiplied reflections of light in the atmosphere. 



1665 What is the use of telescopes f 



They gather together the rays of light, and a greater 

 number are thus brought to the eye. 



1666 How can these rays be gathered together ? 



Rays of light diverge that is, spread out in all direc- 

 tions from a luminous object. The number of these 

 diverging rays which will enter the eye is limited by 

 the size of the pupil. But before they reach the eye, 

 they may be received upon a glass lens of a convex 

 form, which will have the effect of collecting them into 

 a space less in magnitude than the pupil of the eye. If 

 the eye be placed where the rays are thus collected, all 

 the light will enter the pupil. 



The light which produces vision, as will be more fully explained here- 

 after, enters the eye through a circular opening called the pupil, which is 

 the black circular spot surrounded by a colored ring, appearing in the 



