ReflexiUlity of Light. 1 65 



It is, in truth, by reflection that all objects are 

 rendered visible. Even glass, crystal, and water 

 reflect a part of the rays of light, or their forms 

 and substance could not be distinguished ; but 

 those bodies which transmit it copiously, are 

 called clear or transparent ; those which do not 

 transmit it, are termed opake. The whole of the 

 light which falls upon bodies, is not, however, 

 reflected. On the contrary, it is calculated that 

 the smoothest and most polished surfaces do not 

 reflect above half the light that falls upon them. 

 Those bodies with polished surfaces, which re- 

 flect most copiously the rays of light, are called 

 mirrors; by the ancients they were made of 

 metal, as iron, tin, or copper, and exquisitely 

 polished ; those in general use among us are made 

 of glass, rendered opake at the back part by an 

 amalgam or mixture of tin and quicksilver, or 

 mercury. Mirrors are made in various forms; 

 plane, that is, with a smooth and level surface; 

 convex, concave, or cylindrical. The most com- 

 mon are the plane mirrors. 



A ray of light striking perpendicularly, in a 

 direct line, upon a plane mirror, is reflected in 

 exactly the same direction. Those rays which 

 strike it obliquely, are reflected back in an op- 

 posite direction, but^, with exactly the same degree 

 of obliquity. Hence the great law of reflection 

 is, that the angle of reflection's exactly equal to 

 the angle of incidence. This was explained to 



