450

HISTORY OF PHYSICAL ASTRONOMY.

obvious that a mode of accurately determining tlie variations in the ap- 5

parent diameters of the sun and moon, could not fail to lead to an improve- j

ment of the theories of the movements of those bodies. Moreover, the j

invention of the telescope, by revealing the round figures of the planets, j

rendered it desirable to devise some means of measuring small angles witha degree of precision conformable to the existing condition of theoreticalastronomy. The invention of the micrometer, by means of which thisobject was effectually accomplished, is originally due to William Gascoigne ,a young man who was the friend of Horrocks and Crabtree, and who, likethem also, was unfortunately cut off by a premature death in the veryoutset of his career. It will be desirable, however, to give a previousaccount of the successive steps by which the same admirable inventionwas independently effected on the Continent about twenty years after-wards. The person who first suggested the idea of the micrometer in thelatter instance was Huyghens . In his Systema Saturnium, which was |

published in 1659, he remarked that an object placed in the focus of a ]

common astronomical telescope, appears as distinct and as well defined asthe image of a remote body. Hence, in order to determine the apparentdiameter of a celestial object, he inserted a slip of metal of variablebreadth at the focus of the telescope, and observed at what part it exactly j

covered the object. Applying, then, a finely-pointed compass to the slip of \

metal, he measured its exact breadth, and, knowing the focal length |

of the telescope, he hence deduced the apparent magnitude of the )

object. In this manner he determined the apparent diameters of the ;

principal planets.

The Marquis Malvasia, in his “ Epliemerides,” published at Bologna in i1662, describes a method of measuring small angles which may be re- |

garded as the next step in the invention of the micrometer. At the focus |

of the telescope he placed a net-work of fine silver threads at right angles, 1

which formed by their mutual intersection a number of small squares. In ;

order to determine the mutual distances of the threads that were parallel to j

each other, he caused a star situate near the equator to traverse a thread j

perpendicular to their common direction, and counted, by the aid of a pen- |

dulum clock, the times which it took to pass over the successive intervals* . |

The micrometer of Malvasia was a decided improvement of that ofHuyghens . The distances between the threads, when once accurately de-termined, would serve for all future measurements. The method was alsomore generally applicable than that of Huyghens , since, besides the apparentdiameters of the celestial bodies, it might serve to measure the angulardistance of two stars that were at the same time in the field of view of thetelescope. It laboured, however, under the disadvantage that, if the twostars did not appear exactly on the wires, something was unavoidably leftto estimation in the measurement of their mutual distance. Auzout gotrid of this defect by substituting for the reticule of Malvasia two silkthreads, one of which was fixed, while the other was moveable in a direc-tion parallel to it by means of a fine screw. It is evident that, when oncethe distance traversed by the moveable thread during one revolution of §

the screw was ascertained with sufficient accuracy, the angular distance |

between two stars very near to each other might be determined, by causing fthe moveable thread to traverse the interval between the stars, and noting j

the number of revolutions and parts of a revolution of the screw which j

were required to effect this object. I

* Mem. Acad, des Sciences, 1717, p. 78, et seq. j