clear and not dry, the wind must not be east-for from no quarter can it blow less auspicious to astronomers: the eye must have rested from viewing bright objects, such as the moon or an artificial light, for at least half an hour; when the excitement of the retina has ceased, turn the telescope on the planet; a 31-feet achromatic ought to give a fair image of the planet, of the shadow of the ring on the planet, the shadow of the planet on the ring, and of the belt across the body. One satellite may be seen with the 3-feet, but it will not divide the ring; for this to be seen double, a 5-feet achromatic or a 3 or 4-feet Gregorian is the lowest telescope we can use the more powerful the telescope, the more satisfactory will be the view. It must not be supposed, however, that this all-repaying sight is one which can always be commanded, even by the same instrument. The author has known one of Dollond's best 5-feet achromatics fail to shew the division in the ring when not well mounted; proving that the performance of the telescope depends in a great measure upon the stand which carries it; but of this more hereafter.

329. The observer may now venture on the double stars, beginning with those which are easily separated, of which a list will shortly be given. He must find out their position from the celestial globe or maps of the stars; a 3-feet will shew the colours in y Andromeda, the double star Castor; the minute companion of Polaris may be perceived by it; a Lyre will be beyond its power; but Polaris, in Dr. Kitchener's phraseology, "will give of it as good an idea as a kitten does of a cat." If the telescope be mounted only with vertical and horizontal movements, the nebulæ, invisible as they

are to the naked eye, will be pitched upon only by dint of labour and perseverance, even when the declination and right ascension are known. Practice in this case will in time enable an observer to fish them up. Mark, as in the case of the planets, their place on the globe; indeed, 18-inch globes give many of the most remarkable; observe the stars in their neighbourhood; put on the telescope the lowest power possible, which will always, be it remembered, have the largest field of view; catch the nearest star which is visible to the naked eye, and move the instrument in the direction in which the nebula required lies. After some time the probability is, that it will appear in the field of view, when the low power may be exchanged for the one most suited to display its character. Thus the annular nebula in Lyra lies about midway between ẞ and y of that constellation, and may be picked up by moving the telescope from one star to the other. It would facilitate the process much to have an eye-piece adapted to the telescope, which should magnify only about fifteen or twenty times. As such eye-pieces are useful in finding comets when their position is approximately known, they are termed "comet eye-glasses."

To resolve nebulæ into their component stars, light in abundance is requisite-hence the superiority for this purpose of large reflectors with mirrors of eighteen inches at least in diameter. Even large achromatics will resolve but a few. A 3-feet will, however, shew enough of these faint objects to indicate their general character, and of some will give a very satisfactory view: the nebula in Orion, those in Andromeda and Hercules, and some few others hereafter mentioned, will be well

U

worth inspection. A 5-feet achromatic will resolve a few, and only a few, of the nebulous clusters. Still a telescope with a motion only in altitude and azimuth is an unsatisfactory instrument; the constant following of the object by jerks and starts, and the losing of it by the motion of the earth as soon as found, especially under the application of a high magnifying power-the time the instrument takes to become steady after vibrations constantly occurring from the movement, before the vision becomes distinct -all combine to put one out of sorts with this kind of mounting. The superiority of the equatorial mounting-the pleasure of following the object by simply turning the tangent-screw, and the facility with which an unseen object may be identified, will be appreciated by all who are acquainted with this mode of mounting a telescope for celestial observations.

SECTION XIV.

THE EQUATORIAL.

METHOD OF RECTIFYING THE INSTRUMENT-HOW TO FIND AN OBJECT BY ITS RIGHT ASCENSION AND DECLINATION-R.A. AND DECLINATION OF MANY REMARKABLE OBJECTS-THE AUTHOR'S OBSERVATORY-CONCLUSION.

330. If the construction of the universal dial (fig. 52, Plate VIII.), which was explained in § 281, be clearly understood, little remains to be said on the principle of the equatorial. The drawing in Plate IX. represents the German method of mounting a telescope equatorially, and that of the author's Observatory the