LAWS OF EEFLECTION. 429 



of division on the other side, and also the number 

 placed by the side of it. 



When a ray of light meets a surface at a point the 

 line drawn perpendicular to the surface at that point is 

 called ' the perpendicular at the point of incidence ; ' 

 the line from to the centre of the mirror is such a 

 perpendicular for all rays which are incident upon the 

 centre of our mirror. Our experiments prove that 

 light is reflected so that the ray of light makes, after 

 reflection, the same angle with the perpendicular as 

 it made before reflection. This angle, which light 

 incident upon a surface makes with the perpen- 

 dicular, is called the 4 angle of incidence,' while the 

 angle made with the perpendicular after reflection is 

 termed the ' angle of reflection.' Hence, the first law 

 of reflection may be enunciated thus : the angle of re- 

 flection is equal to the angle of incidence. The second 

 law of reflection is usually stated thus : the incident and, 

 the reflected ray, and the perpendicular to the surface at 

 the point of incidence, lie in the same plane. 



The apparatus in fig. 243 is made of a board, cut semi-circular 

 with a saw ; to the straight side a wooden rim is fixed 4 or 5 cm 

 high ; the semicircular rim, having the same height, is made of 

 cardboard. The diameter of the semicircle should be from 30 to 

 60 cm . The strip of cardboard is first cut of larger size than re- 

 quired, placed tightly along the edge of the semi-circular board and 

 fixed temporarily by means of a few tacks, driven to about half 

 their length through the cardboard into the semi-circular edge of 

 the board, and also at both ends of the semicircle into the side of 

 the straight wooden rim. At each end of the semicircle a pencil 

 line is now drawn, so as to mark off precisely the length of card- 

 board required, and this is to be divided into eighteen equal parts. 

 The strip is then taken off" the board, and cut to the required length, 

 leaving at each end, beyond the pencil line, a piece as wide as the 

 thickness of the wooden rim, so that the end of the cariboard may 

 be fixed upon the latter/ : The strip is again straightened, divided, and 



