PORTION OF THE COSMOS. VISUAL POWER. 61 



Huygens of 123 feet focal length, proved entirely to the 

 advantage of the former. Short's costly reflectors were 

 now everywhere adopted, until the successful practical solu- 

 tion, (1759), by John Dollond, of the problem of achro- 

 matism proposed by Leonhard Euler and Klingenstierna, 

 again turned the scale in favour of refractors. The appa- 

 rently incontestable rights of priority of the mysterious 

 Chester More Hall, of Essex (1729), were first made known 

 to the public when John Dollond obtained a patent for his 

 achromatic telescopes. ( 119 ) 



But this victory of the refracting instruments was not 

 of long duration : eighteen or twenty years after the 

 publication of John Dollond' s accomplishment of achro- 

 matism by a combination of crown and flint glass, new 

 fluctuations of opinion were induced by the merited tri- 

 bute of admiration paid, both in England and elsewhere, 

 to the ever memorable labours of a German, William 

 Herschel. The construction of his numerous 7 -feet and 

 20 -feet telescopes, to which magnifying powers of 2200 

 to 6000 could be successfully applied, was followed by the 

 construction of his 40-feet reflector, by means of which 

 were discovered, in August and September 1789, the two in- 

 nermost of the satellites of Saturn, the second, Enceladus ; 

 and soon after Mimas, the first or nearest to the ring. The 

 discovery of the planet Uranus (1781) was made with the 

 7 -feet telescope of Herschel. The satellites of Uranus, 

 which have such feeble light, were first seen by him, in 1787, 

 in his 20-feet instrument arranged for the " front view/' ( 12 ) 

 The previously unattained degree of perfection which this 

 great man was able to impart to his reflecting telescopes, in 

 which the light was reflected only once, led, by the unin- 



