Royal Astronomical Society. 147 



faces as these were, however, produced with some degree of anxiety, 

 much manual labour, and perhaps some admixture of accident, espe- 

 cially in the union of a perfectly parabolic curve with regularity of 

 surface. The superiority of the machine in these respects is so 

 striking as almost to put comparison out of the question. 



" If driven by a steam-engine the manual labour is of course an- 

 nihilated. The control over the machine, by the setting of the 

 cranks, is such, at least with all foci not less than eight diameters 

 of the speculum, that the curve can be changed almost at pleasure 

 from the spherical side to the hyperbolic side of the parabola, and 

 vice versd ; the alterations of the curve being, ceteris paribus, almost 

 exactly commensurate with the adjustments of the cranks. In fact, 

 one of the most anxious and laborious operations is, by this machine, 

 converted into an intensely interesting amusement. With moderate 

 care and a little experience a bad figure never need to be feared, 

 though it may require two or three successive trials to satisfy the 

 feistidiousness of a cultivated and long-practised eye. The lustre of 

 polish transcends even my best efforts by hand, and is the easiest 

 quality of all to obtain ; and however erroneous the figure may be 

 after any unsuccessful effort, the proper curve may be recovered 

 without resorting to the grinder,* or indeed materially impairing the 

 polish — at least, I have not found it needful, even when the differ- 

 ence of foci of the central and exterior portions of a mirror has 

 amounted to fifteen hundredths of an inch. In 3 or 3^ hours by the 

 polisher alone, it is possible to annihilate an error even as enormous 

 as this, I have a strong persuasion that this machine might prove 

 eminently serviceable in working the curves of object-glasses of large 

 dimensions, though of this I have no experience." 



Mr. Lassell then briefly describes the mounting of the telescope, 

 the form, weight, and dimensions of its component parts, and the 

 covering dome. They are in principle almost the same as were used 

 on a smaller scale in his 9 -foot Newtonian. There is a very good 

 model of the dome and mounting, presented by Mr, Lassell, at the 

 apartments of the Society. 



" To afford some notion of the degree of facility attained in the 

 management of so large a dome and telescope, I may mention, that 

 with an assistant I can, without hurry, place an object, invisible to 

 the naked eye, within the field of the telescope in nine or ten 

 minutes from leaving my house. This includes opening the dome, 

 uncovering the large speculum, attaching the eye-piece, setting 

 from the catalogue for the object, and turning the dome to the 

 required azimuth. Without an assistant, I should require three or 

 four minutes longer, which would be principally occupied in open- 

 ing the shutters of the dome. 



" One of the greatest difficulties I have encountered in support- 

 ing the speculum in its various positions equably, is to avoid the 

 effects of the friction of its edge under considerable changes of alti- 

 tude of the telescope. 



" It is obvious, that when the altitude is low, the principal part 

 of the weight of the speculum must be borne upon its edge, and the 



L 2 



