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HISTORY OF PHYSICAL ASTRONOMY.
of the Astronomical Society in two important particulars. In the first place,the secular variation of precession, both in right ascension and declination,is given for each star. Secondly, the proper motions of the stars areassigned with as great a degree of nearness to the truth, as the existing stateof sidereal astronomy allowed. This catalogue has proved of vast ser-vice to astronomers in every part of the civilised world.
An important catalogue has recently issued from the Greenwich Obser-vatory, founded on observations made during the twelve years commencingwith 1830, and ending with 1847. On account of the increasing im-portance of the subject of the proper motions of the stars, it has beendeemed expedient to employ two epochs in the formation of this catalogue.The observations made during the first six years, are reduced to the firstof January, 1840; the epoch of the remaining observations is the com-mencement of 1845. The number of stars in the catalogue is 2156.One peculiarity of this catalogue consists in the constants of reductionbeing computed according to the modification of Bessel’s method, whichMr. Airy had shortly before suggested.
Allusion has been made to Mr. Johnson's catalogue of southern stars.This distinguished astronomer, who was appointed Director of the Ead-cliffe Observatory, Oxford, upon the death of Prof. Rigaud in 1838,has been engaged for several years in re-observing the circumpolar starsof Groombridge’s catalogue. This great undertaking is now all butcompleted. The number of stars observed will exceed the number inGroombridge’s catalogue by about 2000. The epoch of reduction is thebeginning of 1845. A comparison of this catalogue with Groombridge’scannot fail to lead to results of great importance.
In addition to the catalogues of stars alluded to in the foregoing pages,accurate catalogues of a select number of stars have been formed at everyobservatory, both in this country and on the Continent, where meridionalobservations form the chief object of attention. Apart from the specialutility of these catalogues in so far as the practice of observation is con-cerned, they have in many instances been employed in some of the mostdelicate researches of modern astronomy*.
CHAPTER XX.
Early Notions of the Telescope.— Invention of the Telescope in Holland.— Galileoconstructs a Telescope.—Kepler proposes the Telescope composed of Two ConvexLenses.— This Instrument first applied to Astronomical Purposes by Gascoigne.—Telescopic Observations of Huyghens and Cassini.—Reflecting Telescope proposed
by Gregory. — Newton executes a Reflecting Telescope_Efforts of his Successors to
construct these Instruments. — Invention of the Achromatic Telescope by Dollond.—Reflecting Telescopes executed by Herschel.— Modern Improvements in the Re-fracting Telescope . —Improvements in the Construction of Reflecting Telescopes_
The telescope is justly considered to he one of the noblest inventionswhich the annals of human ingenuity can boast of. By its means the
* It may be stated in illustration of this remark, that a small catalogue of stars whichBessel constructed from his own observations at Kbnigsberg, was employed by him incombination with the observations of Bradley and Piazzi, in his celebrated investigationof the quantity and laws ot Precession . Again, M. Otto Struve has availed himself of asmall catalogue of stars founded on observations made at Dorpat, in his recent researcheson the same subject, in connexion with the great problem of the Motion of the SolarSystem in space.