of Saturn; farther off still the ring appears wider and the opening becomes visible; and about seven years before and after 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 some of them were to the discovery of the ring.
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 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).
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
- Substantially the filar micrometer of modern astronomy.