155. To our countryman _William Gascoigne_ (1612?-1644) 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 Marston 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=[95] with which he was able to measure angles of a few seconds. Nothing was known of his discoveries at the time, and it was left for Huygens to invent independently a micrometer of an inferior kind (1658), and for _Adrien Auzout_ (?-1691) 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 _Jean Picard_ (1620-1682). [Illustration: FIG. 67.—Early drawings of Saturn. From the _Systema Saturnium_.] 156. With Gascoigne should be mentioned his friend _Jeremiah Horrocks_ (1617?-1641), who was an enthusiastic admirer of Kepler and had made a considerable improvement 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 of _Philips von Lansberg_ (1561-1632) which Horrocks had verified for the purpose. It was not, however, till long afterwards that Halley pointed out the importance of the transit of Venus as a means of ascertaining the distance of the sun from the earth (chapter X., § 202). It is also worth noticing that Horrocks suggested the possibility of the irregularities of the moon’s motion being due to the disturbing action of the sun, and that he also had some idea of certain irregularities in the motion of Jupiter and Saturn, now known to be due to their mutual attraction (chapter X., § 204; chapter XI., § 243).