220 Hoi^al Astronomical Society. 



step, and forms an epoch in tlie history of the astronomical use of 

 the reflecting telescope. Those only who liave had experience of 

 the annoyance of liaving to keep an object long in view, especially 

 under high magnifying powers, and in micrometrical measurements, 

 with a reflector mounted in the usual manner, having merely an alti- 

 tude and azimuth motion, can duly feel and appreciate the advantage 

 thus gained. But the difficulties to be surmounted in the execution 

 of such a mode of mounting were very considerable — much more so 

 than in the case of an achromatic, — owing partly to the non-coin- 

 cidence of the centre of gravity of tlie telescope and mirror with the 

 middle of the length of the tube, and partly to the necessity of 

 supporting the mirror itself within the tube in a uniform bearing 

 free from lateral constraint, and guaranteed against flexure and 

 disturbance of its adjustment by alteration of its bearings. These 

 difficulties, however, Mr. Lassell overcame : the latter, which is the 

 most formidable, by an ingenious adaptation of the balancing 

 principle first devised, if I am not mistaken, by Fraunhofer and 

 Reichenbach for the prevention of flexure in the tubes of telescopes 

 — a principle which has not received half the applications of which it 

 is susceptible, and which, by throwing the whole strain of the weight 

 of instruments on axes which may be made of unlimited strength, 

 may be employed to destroy the distorting force of gravity on every 

 other part*. 



The success of this experiment was such, and the instrument was 

 found to work so well, that Mr. Lassell cortceived the bold idea of 

 constructing a reflector of two feet in aperture and twenty feet in 

 focal length, and mounting it upon the same principle. The cir- 

 cumstances of his local situation, in the centre of manufacturing 

 industry and mechanical construction, were eminently favourable to 

 the success of this undertaking ; and in Mr. Nasmyth he was fortu- 

 nate enough to find a mechanist capable of executing in the highest 

 perfection all his conceptions, and prepared, by his own love of as- 

 tronomy and practical acquaintance with astronomical observation 

 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 informs us that he visited the Earl of Rosse, 

 at Birr Castle, and besides being favoured with more than one op- 

 portunity of satisfying himself of the excellent performance of that 

 nobleman's three-loot telescope, enjoyed the high privilege of exa- 

 mining the whole machinery for grinding and polishing the large 

 speculum, and returned so well satisfied as to resolve on the imme- 

 diate execution of his own ideas. 



* As, for example, the divided limbs of circles, and the spokes connecting 

 them with their centres; an easy and simple mechanism, which, devised 

 some time ago, and approved by the late M. Bessel, I may, perhaps, take 

 some future opportunity to submit to the Society.— (A^o/e added in the 

 Printing.) 



