During the next half-century we hear of no attempts being made to surpass the large instrument which formed one of the working-tools of Herschel. Then, however, Lord Rosse entered the field, and in the ‘Philosophical Transactions’ for 1840 described a reflector of 3-feet diameter which he had set up at his residence at Parsonstown, Ireland. In 1845 the same nobleman, distinguished alike for his scientific attainments as for his generosity and urbanity of disposition, erected another telescope, the large speculum of which was 6 feet in diameter, 5½ inches in thickness, and its weight 3 tons. Lord Rosse subsequently cast a duplicate speculum of 6 feet and weighing 4 tons. In point of dimensions this instrument far exceeded that of Herschel, and it is still in use, retaining its character as the largest, though certainly not the best, telescope in existence. Its tube is made of 1-inch deal, well bound together with iron hoops; it is 56 feet long and 7 feet in diameter. Mr. Lassell soon afterwards made large specula. He erected one of 2-feet aperture and 20-feet focus at his residence at Starfield, near Liverpool, and in 1861 mounted one of 4-feet diameter and 37-feet focus. This instrument was for some time usefully employed by him at Malta. After Mr. Lassell’s return to England his great telescope remained in a dismantled state for several years, and ultimately the speculum was broken up and “consigned to the crucible of the bell-founder.” It is not a little remarkable that Herschel, Rosse, and Lassell personally superintended and assisted in the construction of the monster instruments with which their names are so honourably associated. In or about the year 1867 a telescope of the Cassegrainian form, and having a metallic speculum 4 feet in diameter and 28-feet focus, was completed by Grubb of Dublin for the observatory at Melbourne. This instrument, which cost something like £14,000, was found defective at first, though the fault does not appear to have rested with the optician. Up to this period specula were formed of a metal in which copper and tin were largely represented. But the days of metal specula were numbered. Leon Foucault, in the year 1859, published a valuable memoir in which he described the various ingenious methods he employed in figuring surfaces of glass to the required curve. He furnished data for determining accuracy of figure. Formerly opticians had considerable trouble in deciding the quality of their newly-ground specula or object-glasses. They found it expedient to mount them temporarily, and then, by actual trial on difficult objects, to judge of their efficiency. This involved labour and occasioned delay, especially in the case of large instruments. Foucault showed that crucial tests might be applied in the workshop, and that glasses could be turned out of hand without any misgivings as to their perfection of figure.