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HISTORY OF PHYSICAL ASTRONOMY.

The observations are made by direct and reflected vision, in the sameway as formerly practised with the mural circle. The instrument hasbeen in constant use since the commencement of the year 1851*.

Although meridional determinations form the principal object of atten-tion at Greenwich, observations of a miscellaneous nature are not altoge-ther neglected. In one of the domes of the more ancient part of thebuilding, there is fitted up an equatorial of 5 feet in diameter, whichwas originally constructed by Ramsden for Sir George Shuckburgh,and which in the year 1811, was presented to the Royal Observa-tory by the Hon. Mr. Jenkiuson. This instrument is occasionally usedin observing comets. It has recently been employed in determining thedistances between Mars and the neighbouring stars, in connexion withthe corresponding observations of the American expedition to Chili f.In the south-eastern dome of the modern part of the observatory, apowerful telescope, mounted equatorially, has been in constant use sincethe year 1838. The object glass, which is by Cauchoix, of Paris , is themunificent gift to the observatory of the Rev. R. Sheepshanks. It hasau aperture of 6.7 inches, and a focal length of 8 feet 2 inches. Thistelescope is fitted with a double-image micrometer, contrived by Mr.Airy. It has been employed in measuring the diameters of the planets,and in determining the distances and angles of position of double stars.An interesting result obtained by the use of this telescope, is the recentdetermination of the true figure of the planet Saturn , by the Rev. Mr.Main, the chief assistant at the Royal Observatory,

Besides the equatorials above referred to, there are also detached tele-scopes for the observation of eclipses, occultations, and other phenomenaof a similar kind. Among the instruments of this description is a reflect-ing telescope of 10 feet, constructed by Sir William Herschel .

In Pond’s time an instrument termed the zenith tube was constructedby Troughton for the observatory, the object of which was to determinesmall distances from the zenith. It consisted of a telescope 25 feet long,adjusted to the zenith by the aid of a plumb-line. The small variationsin' the zenith distance of a star were ascertained by measuring with aninternal micrometer, the deviations of the star from the optical axis ofthe telescope. In practice the use of this instrument was limited toobservations of y JDraconis, which passes within about 2' of the zenith ofGreenwich.

The zenith tube was first erected in 1833, and strong hopes were enter-tained that observations with it would lead to a more accurate determina-tion of the constant of aberration, and might also serve to throw lightupon several other interesting points of astronomy. These hopes, liow»

in the focus of the former telescope will form a distinct image at the focus of the latter.Hence, if the collimating telescope have a fixed position in the meridian, the transit maybe adjusted to it by bringing its cross wires to coincide with the cross wires of the col-limator. The principle of the collimating telescope has been applied to various astrono-mical purposes in modern times by Gauss, Bessel, Kater, and others. We have alreadyhad occasion to mention a singular mode of using it, by Dr. Robinson, in his researcheson Irradiation (see p. 353).

* One of the peculiarities of this instrument, consists in its being made of as few sepa-rate pieces as possible, so as to obviate the effects of unequal expansion, and to securemore effectually the adjustment of the different parts. For this purpose the material ofconstruction used is cast iron, the pivots of the axis being hardened by the process whichengineers term chilling.

See at p. 212 a brief allusion to the object of this expedition.