82 COSMOS. 



feet, led, through the influence of Newton, (in following out 

 the earlier attempts of Mersenne and James Gregory of 

 Aberdeen,) to the adoption, especially in England, of shorter 

 reflecting telescopes. The careful comparison made by Brad- 

 ley and Pond, of Hadley's five-feet reflecting telescopes, with 

 the refractor constructed by Constantin Huygens, (which 

 had, as already observed, a focal length of 123 feet,) fully 

 demonstrated the superiority of the former. Short's expen- 

 sive reflectors were now generally employed until 1759, when 

 John Dollond's successful practical solution of the problem of 

 achromatism, to which he had been incited by Leonhard 

 Euler, and Klingenstierna, again gave preponderance to 

 refracting instruments. The right of priority which appears 

 to have incontestably belonged to the mysterious Chester 

 More, Esq., of More Hall in Essex, (1729,) was first made 

 known to the public, when John Dollond obtained a patent 

 for his achromatic telescopes. 30 



The triumph obtained by refracting instruments was not, 

 however, of long duration. In eighteen or twenty years 

 after the construction of achromatic instruments by John 

 Dollond, by the combination of crown with flint glass, new 

 fluctuations of opinion were excited by the just admiration 

 awarded, both at home and abroad, to the immortal labours 

 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 afterwards, 

 Mimas, the first or the one nearest to the ring. The dis- 

 covery of the planet Uranus in 1781, was made with Herschel's 

 seven-feet telescope, while the faint satellites of this planet 



30 JEdinb. Encyclopedia, vol. xx. p. 479. 



