LESSONS IN PHYSICS. 



73 



such a distance that the two shadows of the rod falling side by 

 side upon the screen shall appear to be of equal darkness, then 

 measure the distance from the flames to the screen, and the 

 amounts of light they give will be as squares of their distance; 

 that is, the one twice as far away gives four times as much light. 



The angle formed in the eye by the rays of light coming from 

 the extremities of an object is called the visual angle, and its size 

 varies with the distance of the object. (See Fig.7 / .) The angle 

 ACB is greater than the angle AEB, and the object seen from 

 C appears larger and nearer than when seen from E. The dis- 

 tance and size of objects are so associated in our minds that we 

 judge of the distance of objects know- 

 ing their size, and knowing their dis- 

 tance we judge of their size. 



The different kinds of vibrations, as 

 sound waves, heat waves, light waves, 

 etc., maybe reflected, but the laws of re- 

 flection which are the same in each case, 

 may be more easily illustrated in con- 

 nection with the study of light. More 

 or less light is reflected from bodies; 

 in fact, we see bodies by reflected light, 

 but some of the metals with smooth 

 surfaces throw back nearly all the light M FIG. 8'. 

 waves and are the best reflectors. In the case of ordinary 

 reflectors or mirrors, the reflecting surface is mercury. In fine 

 telescopic reflectors silver is considered the best material for a 

 reflecting surface. Reflecting surfaces may be plane, concave, or 

 convex. When a beam of light falls on a mirror so as to make a 

 right angle with the surface of the mirror, it is thrown back over 

 the incident path ; but when the beam falls upon the surface so as 

 to form an oblique angle with it, the beam is reflected in some other 

 direction. Suppose a beam of light falls upon a mirror MM at 

 the point A from the direction IA ; it will be reflected in the direc- 

 tion AR. (See Fig. 8'.) Now, if we measure the angles made 

 with the line AP perpendicular to the mirror at A, we find that the 



