HISTORY OF PHYSICAL ASTRONOMY.

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the results of two mural circles. During Mr. Airy’s directorship of theObservatory of Cambridge, he devised a mode of accomplishing the sameobject by the use of only one circle. This practice was adopted by himat Greenwich , soon after his appointment as Astronomer Royal; and, asonly one mural circle was henceforward necessary for that establishment,the mural circle by Jones was dismounted in the year 1839, and wassent to the Cape of Good Hope , for the use of the Royal Observatoryestablished in that colony'.

In practical astronomy, as in every other department of human know-ledge, circumstances are constantly arising which create the necessityfor further improvement. The meridional instruments constructed byTrougliton, for the Royal Observatory of Greenwich , were unrivalledspecimens of artistic excellence, and in the hands of Pond and his suc-cessor, served amply to uphold the reputation of that establishment. Inrecent times, however, it has been found that they are no longer adequateto the exigencies of the present state of astronomical science. Thedifficulty which has been experienced in observing on the meridian thesmall planets that have been recently discovered, and also occasionallyfaint stars, whose positions it is necessary to determine in connexion withextra-meridional observations, has suggested the expediency of replacingthe telescope of the mural circle by one of larger aperture*. The muralcircle, however, not beiug adapted for carrying a large telescope, it be-came necessary to devise a different mode of construction; and as theform of the transit seemed to be most suitable for this purpose, it wasproposed that an instrument should be constructed, which should combinethe functions of both the mural circle and the transit. The advantage of aninstrument capable of assigning icith due precision the two elements of theposition of a celestial body, must be obvious to every reader. The transitcircle devised by Mr. Airy, with a view to the accomplishment of this ob-ject, was constructed by Messrs. Ransome and May, of Ipswich, and Mr.Simms, of London . The circle is 6 feet in diameter. The telescope hasan aperture of 8 inches, and a focal length of 111 feet. The adjustment inazimuth and collimation is effected by means of two collimating telescopes,placed one to the north and the other to the south of the instrumentf.

* The telescope of Trougliton’s mural circle had an aperture of 4 inches, and a mag-nifying power of 150.

t It has been already mentioned (see p. 464) that Roetner determined the error ofcollimation of his transit circle by means of two distinct marks diametrically opposite toeach other. In fixing two objects at the exact distance of 180° apart, he employed atelescope of a peculiar construction, termed the amphioptra, or reciprocal tube, whichhe contrived for that express purpose (Basis Astronomies, p. 97). It is necessary, whena fixed object is employed directly as a meridian mark, that it be situate at a considerabledistance from the telescope, so that the rays of light from it may enter the object glassin sensibly parallel directions, otherwise they will not converge at the focus. The inter-vention of a lens, however, serves to render a near mark equally suitable for this purpose;for since parallel rays of light falling upon the lens converge at its focus, so on the otherhand, if an object be placed at the focus of the lens, the rays of light diverging from itwill be refracted by the lens, and, emerging in parallel directions, will produce the sameeffect as if they came from an object at an infinite distance. Hence, if an object heviewed with a common astronomical telescope, through a lens adjusted to it in this man-ner, it will appear quite distinct in the focus of the telescope, at whatever distance it besituate from the observer. This mode of obtaining the advantage of a distant mark, wasoriginally devised in 1785, by Dr. Rittenhouse, an American philosopher (Trans. Amer.Phil. Soc vol. ii., p. 181). The collimating telescope is an application of the sameprinciple. It consists of a small telescope, having its object glass turned towards theobject glass of the telescope to be adjusted. In this case it is clear that the cross wires

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