REFLECTION OF LIGHT. 377 



from polished surfaces is in accordance with the following 

 laws: 



(1.) The angle of incidence is equal to the angle 

 of reflection. 



(2.) The incident and reflected rays are both in 

 the same plane, which is perpendicular to the 

 reflecting surface. 



(a.) Fill a basin to the brim with mercury or with water blackened 

 with a little ink. In this liquid suspend by a thread a small 

 weight of greater specific gravity than the liquid used ( 253). The 

 plumb-line will be perpendicular to the liquid mirror. Let the 

 plumb-line hang from the middle of a horizontal meter or yard- 



FIG. 279. 



stick. Place the tip of a candle flame opposite one of the divisions 

 of the stick, and place the eye in such a position that the image of 

 the top of the flame will be seen in the direction of the foot of 

 the plumb-line. Mark the point where the line of vision (i. e., the 

 reflected rays) crosses the meter-stick. It will be found that this 

 point and the tip of the flame are equally distant from the middle 

 of the stick. From this it follows (Olney's Geometry, Art. 342) 

 that the angles of incidence and of reflection are equal. 



(&.) Fig. 279 represents a vertical semicircle graduated to degrees, 

 with a background of black velvet. A mirror at the centre is 

 furnished with an index set perpendicular to its plane ; both mirror 

 and index can be turned in any direction desired. A ray of light 

 from any brilliant source is allowed to enter the tube at the base, 

 in the direction of the centre. By means of a little smoke from 

 brown paper, the paths of the incident and reflected rays are easily 

 shown to a large class. 



