On Apparatus for Polishing Specula for Reflecting Telescopes. 107 
revolves with b and works a wheel of 52 attached to the lower | 
surface of the cam d; in the groove of this works a pin fastened 
to the cam-lever e, which is thus made to oscillate ; this is linked 
to the sliding-plate B, which thus by the rotation of the cam is 
made to vibrate with a uniform motion. The plate B carries a 
spindle h, on which vibrates thearm?. The link will either hold 
this arm in position or cause it to vibrate with the eccentric /, 
which is driven by a shaft at the back. The variable crank m can 
be driven either quickly by the pully , or slowly by the worm- 
wheel 0, The pin in m drives the polisher. 
To give Lord Rosse’s motion the pin of 7m is set central, and / is 
set in motion, this gives the stroke, the side motion being given by 
the sliding plate B. To give Mr. Lassell’s, / is stopped and fixed in 
position, so that the pivot of m is equal the distance of the centre 
of the pinion T (fig. 6) from the centre of the speculum, the arm 
7 being held firm by the link /; the pin of m is then set to the dis- 
tance V T (fig. 6), and by the sfitsetttow of B, Mr. Lassell’s eccentric 
motion can be given. 
This machine Mr. Grubb has superseded by one of great 
simplicity of construction. 
Mr. Grubb’s Second Machine. 
In figure 9 you have an isometric perspective view of the 
machine with which the great Melbourne telescope was ground 
and polished. 
A is the speculum in its box revolving on a vertical spindle, B 
the polisher, a portion of the weight of which is counterpoised by 
a lever attached to the bar a. The horizontal bars b,b’ are 
attached to a atc; these are moved by the cranks at dd’, which 
receive their motion (by means of bevelled wheels) from a hori- 
zontal bar connected with the driving pully D. By the adjust- 
ment of the length of the arms b 0’, or of the cranks at dd’, a great 
variety of curves can be given to the bar a, carrying the polisher. 
By means of the handle at e the speculum was made to turn 
on its edge so as to view a distant artificial star, and thus to test 
the figure—thus a great saving of time was effected and the 
. risk of accident diminished. 
