254 Sir J. F. W. Herschel's Address to the 



ments, the ultimate perfection of polish, and the most com- 

 plete absence of local irregularities of surface. The only 

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

 rather say, which seems open to an djoWoW objection, refuta- 

 ble, 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 substance, such as, for instance, 

 earthenware, porcelain biscuit, or slate, would be free from 

 this objection, though possibly open to others of more im- 

 portance. 



Both Mr Lassell and Lord Kosse 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 per- 

 fectly free and equable manner ; but the former has adopted 

 a mode of securing a free bearing on the supports, by sus- 

 pending the mirror, which is a great and manifest improve- 

 ment 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 neces- 

 sity of making provision against these evils, by a mechanism 

 almost identical in principle, I need not dwell upon this 

 point farther than to recommend it to the particular atten- 

 tion of all who may engage in similar undertakings. 



It is right that I should now say something of the perform- 

 ance of the nine-inch and two-feet reflectors. And first, as 

 regards the success of the system of mounting adopted in se- 

 curing the peculiar advantages of the equatorial movement. 

 This appears to have been very complete. The measure- 

 ments, both differential and micrometrical, made with them, 

 and recorded in our Notices, shew that, in this respect, they 

 may be considered on a par with refractors, and in facility of 

 setting and handling they appear nowise inferior. Of the 

 optical power of the former, two facts will enable the meet- 

 ing to form a sufficient judgment. With this instrument Mr 

 Lassell, independently, and without previous knowledge of 



