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 the arm i. The link &will either hold 

 this arm in position or cause it to vibrate with the eccentric l> 

 which is driven by a shaft at the back. The variable crank m can 

 be driven either quickly by the pully n, or slowly by the worm- 

 wheel o. The pin in m drives the polisher. 



To give Lord Rosse's motion the pin of m is set central, and I is 

 set in motion, this gives the stroke, the side motion being given by 

 the sliding plate B. To give Mr. LasseH's, I 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 

 i being held firm by the link k ; the pin of m is then set to the dis- 

 tance Y T (fig. 6), and by the vibration of B, Mr. Lasseli'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 Grubb has superseded by one of great 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 at c ; these are moved by the cranks at d d', 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 b\ or of the cranks at d d\ 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. 



