2O2 A Short History of Asironciny [CH. vm. 



of Saturn; farther off still the ring appeals wider and the 

 opening becomes visible; and about seven years before 

 and after i the periods of invisibility (at A and c) the ring 

 is seen at its widest. Huygens gives for comparison with, 

 his own results a number of drawings by earlier observers 

 (reproduced in fig. 67), from which it may be seen how 

 near son e of them were to the discovery of the ring. 



155. To our countryman William Gascoigne (1612 ?-i644) 

 is due the first recognition that the telescope could be utilised, 

 not merely for observing generally the appearances of celestial 

 bodies, but also as an instrument of precision, which would 

 give the directions of stars, etc., with greater accuracy than 

 is possible with the naked eye, and would magnify small 

 angles in such" a way as to facilitate the measurement 

 of angular distances between neighbouring stars, of the 

 diameters of the planets, and of similar quantities. He was 

 unhappily killed when quite a young man at the battle 

 of Marstcn Moor (1644), but his letters, published many 

 years afterwards shew that by 1640 he was familiar with 

 the use of telescopic " sights," for determining with 

 accuracy the position of a star, and that he had constructed 

 a so-called micrometer * with which he was able to measure 

 angles of a few seconds. Nothing was known of his dis- 

 coveries at the time, and it was left for Huygens to invent 

 independently a micrometer of an inferior kind (1658), and 

 for Adrien Auzout (?-i69i) to introduce as an improvement 

 (about 1666) an instrument almost identical with Gascoigne's. 



The systematic use of telescopic sights for the regular 

 work of an observatory was first introduced about 1667 by 

 Auzout's friend and colleague y^TZ Picard (1620-1682). 



156. With Gascoigne should be mentioned his friend 

 Jeremiah Horrocks (1617 ?-i 641), who was an- enthusiastic 

 admirer of Kepler and had made a considerable improve- 

 ment in the theory of the moon, by taking the elliptic orbit 

 as a basis and then allowing for various irregularities. He 

 was the first observer of a transit of Venus, i.e. a passage 

 of Venus over the disc of the sun, an event which took 

 place in 1639, contrary to the prediction of Kepler in the 

 Rudolphine Tables, but in accordance with the rival tables 



* Substantially the filar micrometer of modern astronomy 



