542 Royal Astronomical Society. 



\vas not so satisfactory. Here, again, Lord Rosse discovered the 

 cause. The triangular levers vphich immediately support the mirror 

 are necessarily not plane, inasmuch as the points which take hold 

 of the plates that adhere to the mirror must project higher (the tele- 

 scope being vertical) than the fulcra of these levers. If, then, when 

 the telescope is lowered, the mirror is allowed to slip edgewise, it 

 throws the lower points of each lever partly out of bearing, and then 

 the mirror is not supported in the way to which its figure is adapted. 

 It was found that some of the points were not in bearing ; and it 

 was also found that, when the telescope was directed to a star, the 

 image was rendered extremely good by screwing or unscrewing to 

 tile proper degree the supports of the hoop or chain. It was found, 

 moreover, that all this adjustment was affected by the bending of 

 the iron base of the fixed frame. On the whole, this part of the 

 mounting is not in a satisfactory state. The Astronomer Royal was 

 not able to say what is the nature of the edge-bearing adopted by 

 Lord Rosse at the present time ; still it is evident that, with due 

 attention, the mounting above-mentioned may be made perfectly 

 available. [The Astronomer Royal has lately been informed, that 

 Lord Rosse has entirely overcome these difficulties, by placing be- 

 tween the back of the mirror and the plates in which the points of 

 the triangular levers act, sheets of tin, which allow the mirror to 

 slip upon the plates, instead of the felt and pitch which formerly 

 united the mirror and the plates.] It may be proper here to men- 

 tion, that Lord Rosse has informed the Astronomer Royal that the 

 mirror which he saw under the polisher has been mounted, and that 

 it shows very well the third star of / Andromedae, — no small proof 

 both of the perfection of the figure and of the efficiency of the sup- 

 port of the mirror in that position. 



The Astronomer Royal then explained his own ideas upon the 

 nature of the mounting, to which (whatever might be the practical 

 difficulties in the mere mechanical operation) he thought it would 

 be necessary to approach. He thought that it was absolutely ne- 

 cessary to give the edge-support by counterpoises ; and this might 

 be done, retaining the present levers, by making the point of each of 

 the small triangles a socket for a ball-and-socket joint, in which 

 turns a lever whose point lodges in the mirror. The extreme 

 hardness of the speculum metal makes it, however, difficult to drill 

 the holes. 



In terminating now the account of the mirror, the Astronomer 

 Royal alluded to the impression made by the enormous light of the 

 telescope ; partly by the modifications produced in the appearances 

 of nebulae figured by Sir John Herschel, partly by the great number 

 of stars seen even at a distance from the Milky Way, and partly 

 from the prodigious brilliancy of Saturn (the only planet which he 

 had an opportunity of seeing). The account given by another 

 astronomer of the appearance of Jupiter was, that it resembled a 

 coach-lamp in the telescope ; and this well expresses the blaze of 

 light which is seen in this instrument. 



The Astronomer Royal then stated that he had had no opportu- 

 nity of trying Mr. Lassell's telescope ; but he had understood from 



