On Apparatus for Polishing Specula for Reflecting Telescopes, 108 
Lord Rosse’s 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 O, which latter drives N and R; on N rests the speculum 
H I, while R 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 R 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, DG 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 dise 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 
