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An axis which points to the pole carries at right angles another axis, on one end of which is the telescope and on the other a counterpoise. The first axis is revolved in the opposite direction to which the earth rotates, by means of an endless screw, turned by a hand movement or driving clock. In the cheaper forms of Equatorials this screw can be thrown in and out of its bearing by an eccentric, but in the better class of instruments, the screw is always in action, and various other means are provided by means of which the telescope can be rapidly moved from one object to another. The second axis merely serves to elevate the telescope to any point below or above the equator, and when following a star is clamped in one position. The two axes give two movements, the first in right ascension and the second in declination. A mount with these movements is called an Angle-block, or Parallactic stand, and when divided circles and reading verniers are added, an Equatorial stand. All Equatorial stands should be provided with adjusting screws, as without them they can only be used at places of exactly the same latitude. When an Equatorial is intended for use at places many degrees distant it should be of the form called universal, in which the polar-axis can be given any altitude between the zenith and horizon.

Mr. Berthon has introduced a modified form of his patent Equatorial specially adapted for refractors and especially useful for those between two and four inches-in aperture, and for portable stands. Being without counterpoises it is exceedingly light, and being made with large hollow bearings is very steady (Fig. 5).

Equatorial stands are mounted in two ways, either on an iron pedestal or on long wooden legs. The former is called the fixed Equatorial and the latter the portable. We have lately made some collapsible legs to a four inch telescope on a portable stand which pack altogether into a comparatively very small box.

REFLECTING TELESCOPES.

The different kinds of reflecting telescopes having already been briefly described, a few remarks on each form, more especially in reference to the necessary adjustments, may be useful. The form of reflector maybe at once known by the curves of the small mirrors. In the Gregorian the small mirror is concave, in the Cassegrainian convex, in the Newtonian flat, in the Herschelian the small mirror is omitted, and in Nasmyth’s modification of the Cassegrainian there are two, convex and flat respectively. The large and small mirrors of all these forms can be made of Speculum metal or glass. The two first are direct and the others indirect vision reflectors.

DIRECT VISION REFLECTORS.

THE GREGORIAN TELESCOPE.

This instrument consists of a large concave mirror which reflects the rays falling upon it to a small concave mirror placed without

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