TELESCOPES. 83 



were first observed by him in 1787, with a twenty-feet "front 

 rific' reflector. 31 The perfection, unattained till then, which 

 this great man gave to his reflecting telescopes, in which light 

 was only once reflected, led, by the uninterrupted labour of 

 more than forty years, to the most important extension of all 

 departments of physical astronomy in the planetary spheres, 

 no less than in the world of nebulas and double stars. 



The long predominance of reflectors was followed, in the 

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

 tion in the construction of achromatic refractors, and helio- 

 meters, paralactically moved by clockwork. A homogeneous, 

 perfectly smooth flint-glass, for the construction of object- 

 glasses of extraordinary magnitude, was manufactured in the 

 institutions of Utzschneider and Fraunhofer at Munich, and 

 subsequently in those of Merz and Mahler ; and in the esta- 

 blishments of Guinand and Bontems, (conducted for MM. Lere- 

 bours 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 Fraunhofer's direc- 

 tions 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; 32 they are both adjusted with 



31 Consult Struve, 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 labours 

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

 the latter at Feldhausen, at the Cape of Good Hope. 



33 See Schumacher's ^sfr-.A T acAr., no. 371 and 611. Cauchoix 



G2 



