LIGHT. 295 



cept from one fifth to one fourth of their light : but when the sun is near the 

 horizon, and the mass of air through wliicla the solar raya pass is consequently 

 vastly increased in thickness, only l-212th part of their light can reach the 

 surface of the earth. If our atmosphere, in its state of greatest density, could 

 be extended rather more than 700 miles from the earth's surface, instead of 

 40 or 50, as it is at present, the sun's rays could not penetrate through it, 

 and our globe would roll on in darkness. Bodies, on the contrary, which 

 are considered as perfectly opaque, will, if made sufficiently thin, allow light 

 to pass through them. Thus, gold-leaf transmits a soft, green light. .i 



643. Liffht, from whatever source it may be 



In what man- t • t • i-j.'l_a. 



ner is light derived, moves, or is propagated in straight 



propagated ? , . , i i • ' • , i 



lines, so long as the medium it traverses is 

 uniform in density. 



If we admit a sunbeam through a small opening into a darkened chamber, 

 the path which the light takes, as defined by means of the dust floating la 

 the air, is a straight line. 



,„^ , ^. , It is for this reason that we are unable to see through a 



What practical 



applications are bent tube, as we can through a straight one. 



made of the jjj taking aim, also, with a gun or arrow, we proceed upon 



movement of o i i o i *- * 



li'^iuin straight the supposition that light moves in straight lines, and try to 

 ^""^^ ^ make the projectile go to the desired object as nearly as pos- 



able by the path along which the hght comes from the object to the eye. 



FiO, 229. 



Thus, in Fig. 229, the hne A B, which represents the line of sight, is also the 

 direction of a line of light passing in a perfectly straight direction from the 

 object aimed at to the eye of the marksman. 



A carpenter depends upon this same principle for the purpose of determin- 

 ing the accuracy of his work. If the edge of the plank be straight and uni- 

 form, the light from all points of its surface will come to the eye regularly and 

 uniformly ; if irregularities, however, exist, they will cause the light to ba 

 irregular, and the eye at once notices the confusion and the point which oc- 

 casions it. 



What is a ray ^44. A ray of light is a line of particles of 

 ofught? light, or the straight line along which light 

 passes from any luminous body. 



A luminous body is said to radiate its hght, because the light issues from 

 it in every direction in straight linoe. 



