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move it in that direction, in the different oblique pofitions of the instrument, the wires are thereby put out of their proper fituation in every other pofition of the inftrument, except when it is in the plane of the meridian; for the equatorial wire should always be parallel to the equator, that the star in paffing over the field of the telescope may move along with it, otherwise one cannot judge whether the telescope be set to the proper declination, except at the inftant the star is brought to the interfection of the wires, which is only a momentary obfervation.

The method I have now put in practice for correcting the refraction of the atmosphere is by applying two lenfes before the object-glass of the telescope; one of them convex, and the other concave; both ground on spheres of the fame radius, which in those I have made is thirty feet. The convex lens is round, of the fame diameter as the object-glafs of the telescope, and fixed into a brass frame or apparatus, which fits on to the end of the telescope. The concave lens is of the fame width, but nearly two inches longer than it is wide, and is fixed in an oblong frame, which is made to flide on the frame the other lens is fixed into, and close to it. Thefe two lenfes being wrought on spheres of the fame radius, the refraction of the one will be exactly deftroyed by that of

VOL. LXIX.

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the other, and the focal length of the object-glass will not be altered by their being applied before it; and if the centers of these two lenfes coincide with each other, and alfo with that of the object-glafs, the image of any object formed in the telescope will not be moved or fuffer any change in its pofition. But if one of the lenfes be moved on the other, in the direction of a vertical arch, fo as to separate its center from that of the other lens, it will occafion a refraction, and the image will change its altitude in the telescope. The quantity of the refraction will be always in proportion to the motion of the lens, fo that by a fcale of equal parts applied to the brass frame, the lens may be fet to occafion a refraction equal to the refraction of the atmosphere in any altitude. If the concave lens be moved downwards, that is, towards the horizon, its refraction will then be in a contrary direction to that of the atmosphere, and the ftar will appear in the telescope as if no refraction had taken place.

There is a small circular spirit level fixed on one fide the apparatus, which ferves to fet it in fuch a position, that the centers of the two lenses may be in the plane of a vertical arch. This level is also used for adjusting a fmall quadrant, which is fixed to it, and divided into degrees, to fhew the elevation of the telescope when directed to the star; then the quantity of refraction an

fwering

fwering to that altitude may be found by the common tables, and the concave lens fet accordingly, by means of the scale at the fide, which is divided into half minutes, and, if required, by using a nonius, may be divided into feconds.

It must be observed, that when a star or planet is but a few degrees above the horizon, the refraction of the atmosphere occafions it to be confiderably coloured. The refraction of the lens acting in a contrary direction would exactly correct that colour, if the diffipation of the rays of light were the fame in glass as in air; but as it is greater in glass than in air, the colours occafioned by the refraction of the atmosphere will be rather more than corrected by thofe occafioned by the refraction of the lens.

A drawing of the refraction apparatus is added, which may ferve to give a more clear idea of it. See plate IV.

EXPLANATION OF THE PLATE.

AA, The circular brass tube, which fits on to the end of the telescope.

BB, The oblong concave lens in its frame, which flides over the fixed convex lens.

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c, The circular spirit level, which fhews when the oblong lens is in a vertical arch.

D, The quadrant to which the fpirit level is fixed, for

shewing the angular elevation of the telescope. E, The milled head fixed to a pinion, by which the

whole apparatus is turned round on the end of the telescope, in order to fet the oblong lens in a vertical arch.

F, Another pinion for fetting the quadrant to the angular elevation of the telescope. By means of these two pinions the air bubble must be brought to the middle of the level.

aa, Is the fcale, with divifions answering to minutes and

half minutes of the refraction occafioned by the concave lens.

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