TELESCOPES. (J3 



The advantages which were at that period supposed to 

 be obtainable only by gigantic length, led great minds, as is 

 frequently the case, to extravagant expectations. Auzout 

 considered it necessary to refute Hooke, who is said to have 

 proposed the use of telescopes having a length of upward of 

 10,000 feet (or nearly two miles), * in order to see animals 

 in the moon. A sense of the practical inconvenience of op- 

 tical instruments having a focal length of more than a hund- 

 red 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 expens- 

 ive 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 re- 

 fracting 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. f 



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 fluctua- 



en tower that had been brought from Marly ; and we also placed them 

 in a tube mounted on a three-sided ladder, a method which, in the dis- 

 covery of the satellites of Saturn, gave us all the success we had hoped.'* 

 — Delambre, Hist, de VAstr. Moderne, torn, ii., p. 785. Optical instru- 

 ments having such enormous focal lengths remind us of the Arabian in- 

 struments of measurement — quadrants with a radius of about 190 feet, 

 upon whose graduated limb the image of the sun was received as in the 

 gnomon, through a small round aperture. Such a quadrant was erect- 

 ed at Samarcand, probably constructed after the model of the older sex- 

 tants of Al-Chokandi (which were about 60 feet in height). Compare 

 Sedillot, Proligomeries des Tables d' Oloug-Beg, 1847, p. lvii. and cxxix. 



* See Delambre,' Hist, de VAstr. Mod., t. ii., p. 594. The mystic 

 Capuchin monk, Schyrle von Rheita, who, however, was well versed 

 in optics, had already spoken in his work, Oculus Enoch et Elim (Antv., 

 1645), of the speedy practicability of constructing telescopes that should 

 magnify 4000 times, by means of which the lunar mountains might bg 

 accurately laid down. Compare also Cosmos vol. ii., p. 323 (note). 



t Edinb. Encyclopedia, vol. xx., p. 479. 



