24 



OX THE CONSTRUCTION AND USE OF 



of polishers of increasinj^ size on the zone a, Fig. 1(), intciinrdiate between the 

 centre and edge, causing their centres to pass along every chord that can be de- 

 scribed tangent to the zone. 



A most perfect and rapid control can thus ])e exercised over a surface, and an 

 uniform result very quickly attained. It becomes a pleasant and interesting occupa- 

 tion to produce a mirror. But two effects have presented tliemselves in this 

 operation, Avhich unfortunately bar the way to the very best nsults. In the first 

 place the edge parts of such mirrors, for more than half an incli all around, bend 

 backwards and become of too great focal Icngtli, and tlic rays from these parts 

 cannot be united with the rest forming the image. In the s(-i:ond place, the sur- 

 foce, wlien critically examined by the second test, is fomid to have a delicate wavy 

 or fleecy appearance, not svni in macliine polishing.' Altliougli the \ariations from 

 the true curve implied l)y tliese latter greatly exaggerated imperfections are ex- 

 ceedingly small, and do not prevent a thermometer bulb in tlie sunsliine appearing 

 like a di.sk surrounded by rings of interference, yet they nuist div(>rt some undula- 

 tions from tlnnr ])ro])er directi(m, or else th(;y woidd not be visible. All kinds of 

 strokes have been tried, straight, sweeping (-ircular, hypocycloidal, (.^c. without 

 effecting their rcmo\al. M. Foucault, who used a pa])er polisher, also encountered 

 them. Eventually they were iinputed to the uneciual ]»ressure of the liand, and in 

 consecjuence a machine to overcome tlie two aljove mentioned faults of manual 

 correction was constructed. 



The miiTor a, is carried by an iron chuck or table h, covered witli a triple 



Fig. 2r,. 



Machine for Local Corrections. 



fold of blanket, and is prevented from slipping off by four elects c c' . Tlic ^•ertical 

 shaft d passes through a worm-wheel c, the I'udless screw of which _/", is driven 

 by a band </, from the primary shaft 7i. At i is the band-wheel for connection to 



' By this it is not meant that there is a ripiilcd polish, like tliat produced by buckskin. 



