On Apparatus for Polishing Specula for Reflecting Telescopes. 101 



The polisher being now prepared the speculum was laid on it, 

 being first gently warmed to about 80°, and moved about on the 

 polisher gently until it was found that the pitch touched it at all 

 points. Rouge and water was then applied and polishing com- 

 menced, the operator proceeding as in grinding, carefully observing 

 that the polishing proceeded equally. 



Tripoli was at first used, but is now entirely superseded by the 

 sesquioxide of iron or Jewellers' rouge, by which name it is sold 

 in the shops. 



Mr. Mudge polished the speculum to a true spherical surfaee 

 and then ended by a few large circular strokes upon the round 

 polisher so as to increase the radius of curvature near the margin. 

 I hope you will not consider that I have gone too much into 

 detail, but as the machines used only change the mode of using 

 the power, I shall not have to recur to these processes again. 



The disadvantages of hand polishing are, the unequal pressure, 

 the inability to exactly control the length and direction of the 

 stroke, the irregular increase of temperature from friction, and finally 

 the inability to work specula larger than eight or nine inches in 

 diameter. 



As already stated, Sir William Herschel was the first who used 

 a machine in polishing, although it is not many years since a 

 description of it was first published. 



We shall now proceed to describe it. 



Sir William HerscheVs Machine. 



The ring RSG surrounded the speculum which rested on the 

 polisher face downwards ; within this ring loosely fitting, and 

 held in position by three pins above and below, there was a thin 

 flat ring T K L, on which was screwed a ratchet ring ; this 

 annulus also carried three cocks at T, K, and L, which rest upon 

 the speculum, with flanges projecting downwards, covered 

 with felt, and capable of being adjusted so as to hold the speculum 

 concentric, yet without being pinched ; R C S is a claw attached 

 to the ring by two pivots at R and S, and with an eye at C, to 

 connect it by a pin to the lever A B, which pivots on B, the 

 power being applied at A. D K and D L are two arms fastened 



I 



