60 SPECIAL RESULTS IN THE URANOLOGICAL 



says, ( 115 ) on terrestrial objects. Auzout, who as early as 

 1663 constructed colossal telescopes without tubes, and, 

 therefore, without a solid or rigid connection between the 

 object-glass and the eye-piece, completed an objective which, 

 with a focal length of 300 feet, would bear a magnifying 

 power of 600. ( 116 ) Dominic Cassini made great use of such 

 object-glasses attached to poles, in the successive discovery, 

 between 1671 and 1684, of the eighth, fifth, fourth, and third 

 of the satellites of Saturn He employed the object-glasses 

 which Borelli, Campani, and Hartsoeker had ground : the 

 latter were of 250 feet focal length. Those of Campani, which 

 enjoyed the highest reputation under the reign of Louis XIV., 

 have been often in my hands at the Paris Observatory during 

 my long residence in that city. If we remember the faint 

 light of the satellites of Saturn, and the difficulty of 

 managing such apparatus, ( 117 ) which could only be moved 

 by the aid of cords, we cannot sufficiently admire the skill 

 and perseverance of the observer. 



The advantages which were then supposed to be attainable 

 exclusively by means of gigantic lengths, led great minds, 

 as is often the case, to form extravagant hopes. Auzout 

 thought it necessary to refute Hooke, who, in order to see 

 animals in the moon, had proposed telescopes 10000 feet, 

 or almost 2 geographical miles, in length. ( 118 ) The 

 practical inconvenience of optical instruments of more than 

 100 feet focal length, gradually led to the introduction, in 

 England especially, and through Newton himself (according 

 to the precedents set by Mersenne and by James Gregory of 

 Aberdeen), of the shorter reflecting instruments. Bradley' s 

 and Pound's careful comparison of 5 -feet Hadleyian re- 

 flectors with the refractor already noticed of Constantiiie 



