TELESCOPES. 81 



worked by strings only,* 8 we cannot sufficiently admire tho 

 skill and the untiring perseverance of the observer. 



The advantages which were at that period supposed to bo 

 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 upwards of 

 10000 feet, (or nearly two miles,) 29 in order to see animals in 

 the moon. A sense of the practical inconvenience of optical 

 instruments having a focal length of more than a hundred 



38 " Nous avons place ces grands verres, tantot sur un grand 

 mat, tantot sur la tour de bois venue de Marly ; enfin nous 

 les avons mis dans un tuyau monte sur un support en forme 

 d'echelle a trois faces, ce qui a eu (dans la decouverte des 

 satellites de Saturne) le succes que nous en avions espeie." 

 " We sometimes mounted these great instruments on a 

 high pole,'' says Dominique Cassini, " and sometimes on the 

 wooden 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 discovery of the satellites of Saturn, 

 gave us all the success we had hoped." Delambre, Hist, de 

 lAstr. moderne, torn. ii. p. 785. Optical instruments having 

 such enormous focal lengths remind us of the Arabian instru- 

 ments of measurement quadrants with a radius of about 190 

 feet, upon whose graduated limb the image of the sun was re- 

 ceived as in the gnomon, through a small round aperture. Such 

 a quadrant was erected at Samarcand, probably constructed 

 after the model of the older sextants of Al-Chokandi (which 

 were about 60 feet in height). Compare Sedillot, ProUgo- 

 menes des Tables d Oloug. Beiqh, 1847, p. Ivii. and cxxix. 



u See Delambre, Hist, de I'Astr. mod., t. ii. p. 594. The 

 mystic Capuchin . Monk, Schyrle von Rheita, who how- 

 ever was well versed in optics, had already spoken in his 

 work, Oculus Enoch et Elite, (Antv. 1645) of the speedy prac- 

 ticability of constructing telescopes that should magnify 4000 

 times, by means of which the lunar mountains might be accu- 

 rately laid down. Compare also Cosmos, vol. ii. p. 705 (note). 



VOL. III. O 



