52 COSMOS. 



the graduation of the arc would have failed entirely, or to a 

 considerable extent, in affording that greater precision of 

 observation at which it aimed, if optical and astronomical 

 instruments had not been brought into accord, and the cor- 

 rectness of vision made to correspond with that of measure- 

 ment. The micrometer- application of fine threads stretched 

 in the focus of the telescope, to which that instrument owes 

 its real and invaluable importance, was first devised, six years. 

 afterwards (1640), by the young and talented Gascoigne. 3 



While, as I have already observed, telescopic vision, obser- 

 vation, and measurement, extend only over a period of about 

 240 years in the history of astronomical science, we find, 

 without including the epoch of the Chaldeans, Egyptians, and 

 Chinese, that more than nineteen centuries have intervened 

 between the age of Tiinochares and Aristillus 4 and the dis- 

 coveries of Galileo, during which period the position and course 

 of the stars were observed by the eye alone, unaided by instru- 

 ments. When we consider the numerous disturbances which 

 during this prolonged period checked the advance of civiliza- 

 tion, and the extension of the sphere of ideas among the nations- 

 inhabiting the basin of the Mediterranean, we are astonished 

 that Hipparchus and Ptolemy should have been so well 

 acquainted with the precession of the equinoxes, the com- 

 plicated movements of the planets, the two principal inequa- 

 lities of the moon, and the position of the stars ; that Coper- 



3 The unfortunate Gascoigne, whose merits remained so 

 long unacknowledged, lost his life, when scarcely twenty- 

 three years of age, at the battle of Marston-Moor, fought by 

 Cromwell against the royalists. See Derham in the Philos. 

 Transact., vol. xxx. for 1717-1719, pp. 603-610. To him 

 belongs the merit of a discovery which was long ascribed to 

 Picard and Auzout, and which has given an impulse pre- 

 viously unknown to practical astronomy, the principal object 

 of which is to determine positions in the vault of heaven. 



4 Cosmos, vol. ii. p. 544. 



