BOOK HI. 



APPLICATIONS OF THE PHENOMENA AND THE 

 LAWS OF LIGHT. 



CHAPTER I. 



MIRRORS AND REFLECTING INSTRUMENTS. 



I. MIRRORS OF POLISHED METAL SILVERED MIRRORS REFLECTORS. 



THE use of mirrors is very ancient. Without going back to the 

 time of Moses, and the book of Exodus which refers to the 

 mirrors of the women who stood at the door of the tabernacle, metal 

 mirrors were in use among the ancient Egyptians (Fig. 139). In 

 Greece and Rome, people decorated the walls of their rooms with 

 polished and reflecting plates of steel, silver, gold, and obsidian ; 

 it appears too, if one may judge by different passages in Pliny and 

 Aristotle, that glass mirrors lined with a sheet of polished metal 

 were not unknown. 



But it was not until the fifteenth century that we find silvered 

 plates of glass substituted for polished metal. In the present day we 

 know how universally they are used either for the toilet or for 

 interior or exterior ornamentation. Although glass mirrors are in- 

 conveniently breakable, they are still greatly superior to metal ones, 

 as they do not get dim, whilst the former rapidly oxidize and tarnish 

 and thus require careful keeping in order. 



In the present day, manufacturers of looking-glasses can produce 

 them of enormous size and with such delicacy of polish that it equals 

 the beauty of the transparent substance itself. The whiter and more 



