Royal Astronomical Society. 253 



vation and with the construction of specula, to give them their 

 full effect. It was of course, however, the construction and 

 polishing of the large reflector which constituted the chief 

 difficulty of this enterprise. To ensure success, Mr Lassell 

 spared neither pains nor cost. As a preliminary step, he in- 

 forms us that he visited the Earl of Rosse, at Birr Castle, 

 and besides being favoured with more than one opportunity 

 of satisfying himself of the excellent performance of that 

 nobleman ""s three-foot telescope, enjoyed the high ])rivilege of 

 examining the whole machinery for grinding and polishing 

 the large speculum, and returned so well satisfied as to re- 

 solve on the immediate execution of his own ideas. 



The mode of casting and grinding the mirror, differing in 

 some of the details, though proceeding generally on the same 

 principle as Lord Rosse's {i. e.,by a chilled casting), has been 

 described in a communication read to this Society on the 8th 

 of December last. The polishing was performed on a ma- 

 chine almost precisely similar to that of his Lordship. But 

 finding, after many months' trial, that he could not succeed 

 in obtaining a satisfactory figure, he was led to contrive a 

 machine for imitating, as closely as possible, those evolutions 

 of the hand by which he had been accustomed to produce 

 perfect surfaces on smaller specula. This machine has been 

 described (and a model of it, as well as Mr Nasymth's finished 

 working drawings of it, exhibited) in a paper of great inte- 

 rest read at the last meeting of this Society, of which also 

 an abstract has been printed in our Notices, and must by 

 this time be in the hands of every Fellow here present, so 

 that it cannot be necessary for me to recapitulate its con- 

 tents. Suffice it to say, that I have carefully examined both 

 the drawings and the model, and having myself had some ex- 

 perience in the working and polishing of reflecting specula, 

 approaching (though inferior) in magnitude to Mr LasselPs, I 

 am enabled to say, that it seems to unite every requisite for 

 obtaining a perfect command over the figure ; and when exe- 

 cuted with that finish which belongs to every work of Mr 

 Nasmyth, from the steam-hammer down to the most delicate 

 product of engineering and mechanical skill, cannot fail to 

 secure, by the oily smoothness and equability of its move- 



