CHAP, i.] MIRRORS AND REFLECTING INSTRUMENTS. 203 



Silver, as well as- tin, lias a considerable reflecting power, but the tint 

 of the image is slightly yellowish. 



In Belgium and many other northern countries, mirrors having a 

 movable joint are placed outside the windows of houses and arranged 

 in such a way as to enable what is passing outside to be seen in the 

 interior of the room. These mirrors are also used by shopkeepers 

 to watch their goods, and are known by the name of " spies." 



FIG. 140. - Venetian mirror. 



Large silvered or metal mirrors are also used to reflect light from 

 the sky to the interior of a room, which would otherwise be dark. 

 These reflectors are frequently employed in dark and narrow streets 

 in London and other large towns. 



Before describing the scientific instruments based on the pheno- 

 menon of reflection from the surface of plane mirrors, we may 



