INTRODUCTION TO PHYSIOLOGICAL OPTICS 405 



the hole in the cork. Put back the cork. Place 

 the incense-holder in the optical box and put the 

 glass lid on the box. Arrange the lantern to 

 throw a beam of light through the window into 

 the box. The smoke will be made luminous by 

 the light so that the path of the rays can be seen. 

 Make the rays parallel by pushing in the draw- 

 tube holding the outer projecting lens of the 

 lantern. "' Set the plane mirror against the side 

 of the box. Let the rays fall obliquely upon 

 the mirror. Accurate measurement would show 

 (1) that the incident ray, the reflected ray, and 

 the perpendicular to the point of incidence, all 

 lie in the same plane, and (2) that the angle, 

 between the incident ray and the perpendicular 



angle of incidence is equal to the angle 

 between the perpendicular and the reflected ray 



angle of reflection (Hero of Alexandria, about 

 100 B. C.). 



REFLECTION FROM CONCAVE MIRRORS 



Principal Focus. 1. Place the concave mirror 

 (the polished inner surface of the segment of a 

 sphere of 5 cm. radius) at right angles to the 

 pencil of parallel rays. 



The rays will be reflected to a point 2 J cm. 

 from the mirror. 1 This point, to which parallel 



1 Much smoke will make the rays less visible. The incident 



