On Apparatus for Polishing Specula for Reflecting Telescopes, 103 

 Lord Rosses Machine, 



Fig. 4. 



In this machine the speculum is worked face upwards, both 

 grinder and polisher being grooved so that an even distribution of 

 the emery used in grinding is obtained as well as of the rouge in 

 polishing. The polisher fits loosely but accurately in the ring, so 

 that it revolves with the speculum, but at a different rate, partly 

 from being carried round with it by friction at the end of each 

 stroke, and partly from the change of direction given to the stroke 

 each time by the revolution of the eccentric. 



The power is applied by means of the spindle A, which drives the 

 spindle P, carrying the eccentric B ; this eccentric gives the length 

 of stroke, equal about one-third the diameter of speculum. P again 

 drives 0, which latter drives N and R; on N rests the speculum 

 H I, while It carries the back eccentric G, which controls the side 

 motion, usually about one-fifth of the diameter (measured on the 

 side of the speculum). The diameters of the various pulleys, as 

 now used for polishing the three feet speculum, are approximately 

 as follows : — Those on the spindle P, 30 and 7 (inches) ; O, 18, 9, 

 and 18 ; N 36, and It 30 ; G performs a revolution in about five 

 strokes of the eccentric B, and the speculum once in about eleven ; 

 D is a fixed guide, D G being rigidly attached to the ring K L, 

 carrying the polisher. This bar and polishing ring is supported 

 at D, and by the fork at G, which latter is free to revolve in its 

 socket. The polisher is counterpoised, leaving a weight of about 

 10 lbs. pressing on the speculum, M being a circular disc attached 

 to one end of the lever by its centre, but by six hooks in its 

 circumference to the polisher. 



The curve actually described by the centre of the polisher in 

 the model of Lord Rosse's machine which you see before you, of 



12 



