REFLECTION OF LIGHT. 303 



whatismeant 658. An image, in optics, is the figure of 

 by an image ? ^jjy object made by rays proceeding from the 

 several points of it. 



Tvhatisacom 659. A common looking-glass consists of a 

 ^assT *"" '°°' glass plate, having smooth and parallel sur- 

 faces, and coated on the back with an amalgam* of tin 

 and quicksilver. 

 „ ,^ . The images formed in a common looking-jrlass 



How are the im- ^ " . 



ages formed in arc mainlv Droduccd by the reflection of the 



* looking-glass 7 n \ r 



rays of light irom the metallic surface attached 

 to the back of the glass, and not from the glass itself. 



The effect may be explained as follows: — A portion of the light incident 

 upon the anterior surface is regularly reflected, and another portion irregu- 

 larly. The first produces a very faint image of an object placed before the 

 glass, whUe the other renders the surface of the glass itself visible. Another, 

 and much greater portion, however, of the liglit falling upon the anterior sur- 

 face passes into the glass and strikes upon the brilliant metallic coating upon 

 the back, from which it is regularly reflected, and returning to the eye, pro- 

 duces a strong image of the object. There are, therefore, strictly speaking, 

 two images formed in every looking-glass — the first a faint one by the light 

 reflected regularly from the anterior surface, and the second a strong one by 

 the light reflected from the metaUic surface ; and one of these images will be 

 before the other at a distance equal to the thickness of the glass. In good 

 mirrors, the superior brilliancy of the image produced by the metallic surfaca 

 will render the faint image produced by the anterior surface invisible, but in 

 glasses badly sQvered, the two images may be easily seen. 



If the surfaces of the mirror could be so highly polished as to reflect regu- 

 larly all the light incident upon it, the mirror itself would be invisible, and the 

 observer, receiving the reflected light, would perceive nothing but the images 

 of the objects before it. This amount of polish it is impossible to effect arti- 

 ficially, but in many of the large plate-glass mirrors manufactured at the pres- 

 ent time, a high degree of perfection is attained. Such a mirror placed ver- 

 tically against the wall of a room, appears to the eye merely as an opening 

 leading into another room, precisely similar and similarly furnished and illum- 

 inated ; and an inattentive observer is only prevented from attempting to 

 w^alk through such an apparent opening by encoiintering his own image iS 

 he approaches it. 



660. A T)lane mirror only chanjojes the direc- 



In what man- /. , /• i • i i • i . n 



ner does a tiou 01 tlic rays 01 liglit wliich tail upon it, 



plane minor .*^ ,.,. . . ' 



reflect rays of without altering their relative position. If 

 '' they fall upon it perpendicularly, they will be 



* An amalgam is a mixture or compound of quicksilver and some ether metaL 



