796 



THE SENSE OF SIGHT 



A beam of light falling upon an unpolished surface suffers a reflec- 

 tion of its rays in all directions, because inasmuch as the surface is 

 composed of projecting particles which receive incident rays at all 

 angles, the reflected rays miist be diffused or scattered in all directions. 

 Naturally, the intensity of the reflected light is always less than that 

 of the incident light, because at least some of the original vibrations 



are converted into vibrations of the 

 reflecting surface. Thus, the intensity 

 of the reflected light really depends 

 upon, (a) the brilliancy of the source of 

 light, (6) the perfection of the polish, 

 (c) the angle of the incident ray, {d) 

 the character of the reflecting sub- 

 stance, and (d) the character of the 

 medium in which the reflection is tak- 

 ing place. 



Fig. 404. — Reflection from 

 Plane Mirrors. 



In accordance with their shape, reflecting 

 surfaces may be classified as plane, concave, 

 convex, spherical, parabolic, conical, etc. 

 The reflection from a plane mirror is illustrated 

 by Fig. 404. If a ray of light emitted by 

 point D, meets the surface CF at the angle DBAj the reflected ray forms the angle 

 EBA. The eye at E then sees the image of D as if it were placed at /, this point 

 being situated where the prolongation of EB intersects the perpendicular drawn 

 through D. Hence, the determination of the position and size of images formed 

 by plane mirrors, resolves itself into a determination of the image points of the 

 several different luminous points. It will be seen, therefore, that the image is 

 perceived as being located behind the mirror at a distance equal to that of the 

 given points. 



Fig. 405. — Reflection from a Concave Spherical Mirror if Its Incident Ray is 



Parallel. 



Spherical mirrors are those possessing the curvature of a sphere, 

 and are formed, therefore, by the revolution of an arc around the radius 

 CD. The inner concave and the outer convex surface may of course 

 be supposed to be made up of an infinite number of plane mirrors. 

 Reflection may take place from the former as well as from the latter. 

 Cj the center of the hollow sphere, constitutes the geometrical center 

 or center oj curvature, while a line drawn through C and D, forms the 



