Royal Astronomical Societi/, 221 



The mode of casting and grinding the mirror, differing in some 

 of the details, though proceeding generally on tlie same principle as 

 Lord Rosse's (t. 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 machine 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 evolu- 

 tions of the hand by which he had been accustomed to produce per- 

 fect surfaces on smaller specula. This machine has been described 

 (and a model of it, as well as Mr. Nasmyth's finished working 

 drawings of it, exhibited) in a paper of great interest read at the 

 la/.t meeting of tliis 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 contents. Suffice it to say, that I have carefully 

 examined both the drawings and tlie model, and having myself had 

 some experience in the working and polishing of reflecting specula, 

 approaching (though inferior) in magnitude to Mr. Lassell's, 1 am 

 enabled to say that it seems to unite every requisite for obtaining 

 a perfect command over the figure ; and when executed 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 movements, the ultimate perfection of polish, 

 and the most complete absence of local irregularities of surface. 

 The only part which 1 do not quite like about it, or perhaps I should 

 rather say which seems open to an a priori objection, refutable, and, 

 in point of fact, refuted by the practical results of its operation, is 

 the wooden polisher, owing to the possibility of warping should 

 moisture penetrate the coating of pitch with which it is (I presume) 

 enveloped on every side. Some unhygrometric, non-metallic sub- 

 stance, such as for instance earthenware, porcelain biscuit, or slate, 

 would be free from this objection, though possibly open to others of 

 more importance. 



Both Mr. Lassell and Lord Rosse appear to be fully aware of 

 the vital importance of supporting the metal, not only while in use, 

 but also while in process of polishing, in a perfectly free and 

 equable manner ; but the former has adopted a mode of securing a 

 free bearing on the supports, by suspending the mirror, which is a 

 great and manifest improvement on the old practice of allowing it 

 to rest on its lower edge, by which not only is the figure necessarily 

 injured by direct pressure, but the metal is prevented from playing 

 freely to and fro, and taking a fair bearing on its bed. As I have, 

 however, on another occasion enlarged on the necessity of making 

 provision against these evils, by a mechanism almost identical in 

 principle, I need not dwell upon this point further than to recom- 

 mend it to the particular attention of all who may engage in similar 

 undertakings. 



