The Achromatic Telescope. 453 



1807. He now began to distinguish, himself. Since lenses, 

 however accurately ground, are very apt to have the truth of 

 their surfaces injured in polishing, he devised a machine, by 

 which the figure is so preserved and corrected to the last, that 

 a perfect result is secured, even in the hands of ordinary 

 workmen ; an invention subsequently employed independently 

 by Ross, and at present by his son-in-law, Dallmeyer, in our 

 own country. He reconstructed Guinand's furnaces, and after 

 great trouble and labour succeeded in obtaining glass of purer 

 quality and larger dimensions, as well as in deducing formulas 

 for determining the most efficient curves. The Optical Insti- 

 tute, which in 1817 had been removed to Munich, was placed 

 under his control, and he continued for a series of years to 

 direct its operations, with latterly 50 workmen under him ; and 

 to superintend the construction of object-glasses, such as up 

 to that time the world had never seen, and of which he said 

 that the difficulty of making them equally good with smaller 

 ones was greater than in the proportion of the cube of the 

 aperture. At length his health, which had never been strong, 

 and might, it was thought, have suffered from the intense heat 

 of his furnaces, gave way, and he was cut off by pulmonary 

 disease, at the early age of 40, June 7, 1826, alike respected 

 in private, and distinguished in public life. His most cele- 

 brated work was the great Dorpat telescope, 9 k inches in aper- 

 ture, the prime cost of which was £950 ; but he was engaged on 

 one of nearly 13 inches at the time of his death. The estab- 

 lishment passed into the hands of Merz, by whom its 

 reputation has been sustained, in part at least, for it is said 

 that the old Frauenhofer achromatic is still considered un- 

 rivalled. The extent of its operations is something surprising. 

 It was stated on good authority that three or four years ago 

 Merz had from £25,000 to £30,000 worth of large object-glasses 

 (i.e., from 4i inches upwards) in stock, besides " heaps " of 

 smaller ones. Of late years, the superiority of Munich work 

 has been questioned by the possessors of English and American 

 telescopes ; the material is undoubtedly more liable to decom- 

 position than Chancers glass ; and even in point of size they 

 seem to have fallen behind. Frauenhofer was willing to under- 

 take an aperture of 18 inches, but we have not heard of the 

 construction of anything so large. The two largest from that 

 workshop which have attained any celebrity, are those of 

 Poulkova and Harvard College ; a third of that size was less 

 successful; one of 16 inches has since been spoken of. But 

 these have been surpassed by Alvan Clark, in America, 18 1 

 inches; by Porro, in Paris, 2 If inches; and by Buckingham 

 and Cooke, in England, 20 and 25 inches respectively (the 

 latter not yet completed). 



