156 DISSERTATION SECOND. ; [parti. 



but Hevelius did not think that the other point, viz. the 

 optical centre of the object glass, was equally well defined. 

 This doubt, however, might have been removed by a di- 

 rect appeal to experiment, or to angles actually measured 

 on the ground, firs*t by an instrument, and then by trigono- 

 metrical operations. From thence it would soon have been 

 discovered, that the centre of a lens is in fact a point de- 

 fined more accurately than can be done by any mechanical 

 construction. 



This method of deciding the question was not resorted 

 to. Hevelius and Hooke had a very serious controversy 

 concerning it, in which the advantage remained with the lat- 

 ter. It should have been observed that the French astro- 

 nomer, Picard, was the first who employed instruments fur- 

 nished wilh telescopick sights, about the year 1665. It ap- 

 pears, however, that Gascoigne, an English gentleman, who 

 fell at the battle of Marston-moor, in 1644, had anticipated 

 the French astronomer in this invention, but that it had 

 remained entirely unknown. He had also anticipated the 

 invention of the micrometer. The vast additional accuracy 

 thus given to instruments formed a new era in the history 

 of astronomical observations. 



Though Galileo had discovered the satellites of Jupiter, 

 their times of revolution, and even some of their inequali- 

 ties, it yet remained to define their motions with precision, 

 and to construct tables for calculating their places. This task 

 was performed by the elder Cassini, who was invited from 

 Italy, his native country, by Louis the Fourteenth, and 

 settled in France in 1669. His tables of the satellites had 

 been published at Bologna three years before, and he con- 

 tinued to improve them, by a series of observations made 

 in the observatory at Paris, with great diligence and accu- 

 racy. 



