Lord Rosse’s 6-ft. reflector has now been used for nearly half a century, and its results ought to furnish us with good evidence as to the value of such instruments. It has done important work on the nebulæ, especially in the re-observation of the objects in Sir J. Herschel’s Catalogues of 1833 and 1864. To this instrument is due the discovery of spiral nebulæ; and perhaps this achievement is its best. But when we reflect on the length of its service, we are led to wonder that so little has been accomplished. For thirty years the satellites of Mars eluded its grasp, and then fell a prize to one of the large American telescopes. The bright planets[5] have been sometimes submitted to its powers, and careful drawings executed by good observers; but they show no extent of detail beyond what may be discerned in a small telescope. This does not necessarily impugn the figure of the large speculum, the performance of which is entirely dependent upon the condition of the air. The late Dr. Robinson, of Armagh, who had the direction of the instrument for sometime, wrote in 1871:—“A stream of heated air passing before the telescope, the agitation and hygrometric state of the atmosphere, and any differences of temperature between the speculum and the air in the tube are all capable of injuring or even destroying definition, though the speculum were absolutely perfect. The effect of these disturbances is, in reflectors, as the cube of their apertures; and hence there are few hours in the year when the 6-foot can display its full powers.” Another of the regular observers, Mr. G. J. Stoney, wrote in 1878:—“The usual appearance [of the double star γ^2 Andromedæ] with the best mirrors was a single bright mass of blue light some seconds in diameter and boiling violently.” On the best nights, however, “the disturbance of the air would seem now and then suddenly to cease for perhaps half a second, and the star would then instantly become two very minute round specks of white light, with an interval between which, from recollection, I would estimate as equal to the diameter of either of them, or perhaps slightly less. The instrument would have furnished this appearance uninterruptedly if the state of the air had permitted.” The present observer in charge, Dr. Boeddicker, wrote the author in 1889:—“There can be no doubt that on favourable nights the definition of the 6-foot is equal to that of any instrument, as is fully shown by Dr. Copeland’s drawings of Jupiter published in the ‘Monthly Notices’ for March 1874. It appears to me, however, that the advantage in going from the 3-foot to the 6-foot is not so great in the case of planets as in the case of nebulæ; yet, as to the Moon, the detail revealed by the 6-foot on a first-class night is simply astounding. The large telescope is a Newtonian mounted on a universal joint. For the outlying portions of the great drawing of the Orion nebula it was used as a Herschelian. As