24 



ox THE CONSTRUCTION AND USE OF 



of polishers of increasing size on the zone a, Fig. 16, intermediate 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 be 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 themselves in this 

 operation, which unfortunately bar the way to the very best residts. In the first 

 place the edge parts of such mirrors, for more than half an inch all aroimd, bend 

 backwards and become of too great focal length, and the rays from these parts 

 cannot be united with the rest forming the image. In the second place, the sur- 

 face, when critically examined by the second test, is found to have a delicate wavy 

 or fleecy appearance, not seen in machine polishing.* Although the variations from 

 the true curve implied by these latter greatly exaggerated imperfections are ex- 

 ceedingly small, and do not prevent a thermometer bidb in the sunshine appearing 

 like a disk surrounded by rings of interference, yet they must divert some imdula- 

 tions from their proper direction, or else they would not be visible. AU kinds of 

 strokes have been tried, straight, sweeping circular, hypocycloidal, &c. without 

 effecting their removal. M. Foucault, who used a paper polisher, also encountered 

 them. Eventually they were imputed to the unequal pressure of the hand, and in 

 consequence a machine to overcome tlie two above mentioned faults of manual 

 correction was constructed. 



The mirror a, is carried by an iron chuck or table h, covered Avith a triple 



Fig. 25. 



Macliine for Local Corrections. 



fold of blanket, and is prevented from slipping off by four elects c c'. The vertical 

 shaft d passes through a worm-wheel e, the endless screw of Avhich /, is driven 

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



By this it is not meant that there is a rippled polish, like that produced by bueliskin. 



