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EQUATORIAL STANDS.

Reflecting telescopes are mounted on several forms of equatorials, a few of which require description. The first is the cheapest form which can be made, and is suitable for telescopes up to 6 or 8 inches in aperture, especially those of a portable nature. The second is on Mr. Berthon’s plan, and is suitable either for fixed or portable instruments of 6 to 8 inches aperture; its great advantage is its remarkable steadiness. The last to be noticed is that known as the Berthon equatorial, and is specially suitable for large telescopes, and when clockwork motion is to be applied. The possessor of a io or 12 inch mirror on a stand of this form, with a driving clock, will rival in instrumental power some of the best observatories.

HORNE AND THORNTHWAITE’S PORTABLE EQUATORIAL. (Fig. 21.)

Until lately, equatorials of even very moderate aperture were only in the possession of few observers ; being of a costly nature they were confined to the observatories of gentlemen whose means equalled their love for scientific pursuits. It was therefore thought that the introduction of a telescope of considerable power on a portable stand, and at a very moderate price, would be a great boon to those observers who have not the space or means to erect an expensive observatory. In order to supply this want, the above-mentioned instrument is manufactured.

The optical parts consist of a 6 inch silvered-glass mirror of about 46 inches focus, guaranteed to divide stars 1" apart, on favourable occasions, or even closer; two achromatic eye-pieces and a Barlow’s lens, or if desired an extra eye-piece in place of the Barlow’s. The stand is so constructed that it combines steadiness and portability. There is a tangent screw movement in right ascension. The right-ascension and declination circles are fully divided, with verniers. We have lately introduced a new method of clamping these equatorials which possesses several advantages. Every necessary adjustment is provided for both the optical portions and stand. The entire body can be revolved so that the eye-piece can always be brought to an easy position for observation ; it can also be readily separated from the stand. If the body-tube is removed and taken indoors, the counterpoise may be at the north of the foot, and a waterproof cover thrown over the entire stand; it can thus be left out of doors, and a considerable amount of such exposure will not injure the. movements. This will of course be a great advantage over setting an instrument in equatorial position on every occasion of observing with it. This equatorial can be made to suit any latitude, and can also be obtained without divided circles, at a reduced price (Fig. 20). Larger forms of this equatorial are made to suit telescopes of 6 or 8 inches in aperture and longer foci.

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