tions of opinion were excited by the just admiration award- 

 ed, both at home and abroad, to the immortal labors of a 

 German, William Herschel. The construction of numerous 

 seven-feet and twenty-feet telescopes, to which powers of 

 from 2200 to 6000 could be applied, was followed by that of 

 his forty-feet reflector. By this instrument he discovered, in 

 August and September, 1789, the two innermost satellites 

 of Saturn Enceladus, the second in order, and^soon after- 

 ward, Mimas, the first, or the one nearest to the ring. The 

 discovery of the planet Uranus in 1781 was made with 

 Herschel's seven-feet telescope, while the faint satellites of 

 this planet were first observed by him in 1787, with a twen- 

 ty-feet "front view" reflector.* The perfection, unattained 

 till then, which this great man gave to his reflecting tele- 

 scopes, in which light was only once reflected, led, by the 

 uninterrupted labor of more than forty years, to the most 

 important extension of all departments of physical astron- 

 omy in the planetary spheres, no less than in the world of 

 nebulae and double stars. 



The long predominance of reflectors wus followed, in the 

 earlier part of the nineteenth century, by a successful emu- 

 lation in the construction of achromatic refractors, and heli- 

 ometers, paralactically moved by clock-work. A homoge- 

 neous, perfectly smooth flint glass, for the construction of 

 object-glasses of extraordinary magnitude, was manufactured 

 in the institutions of Utzschneider and Fraunhofer at Mu- 

 nich, and subsequently in those of Merz and Mahler ; and in 

 the establishments of Guinand and Bontems (conducted for 

 MM. Lerebours and Cauchoix) in Switzerland and France. 

 It will be sufficient in this historical sketch to mention, by 

 way of example, the large refractors made under Fraunho- 

 fer's directions for the Observatories of Dorpat and Berlin, 

 in which the clear aperture was 9' 6 inches in diameter, with 

 a focal length of 14 '2 feet, and those executed by Merz and 

 Mahler for the Observatories of Pulkowa and Cambridge, in 

 the United States of America ;t they are both adjusted with 



* Consult htruve, Etudes d'Astr. Stellaire, 1847, note 59, p. 24. I 

 have retained the designations of forty, twenty, and seven-feet Herschel 

 reflecting telescopes, although in other parts of the work (the original 

 German) I have used French measurements. I have adopted these 

 designations not merely on account of their greater convenience, but 

 also because they have acquired historical celebrity from the important 

 labors both of the elder and younger Herschel in England, and of the 

 latter at Feldhausen, at the Cape of Good Hope. 



t See Schumacher's Astr. Nachr., No. 371 and 611. Cauchoix and 



