FIG. 13.—FOUR-FOOT REFLECTOR EQUATORIALLY MOUNTED.
The mountings of the reflector are in principle precisely similar to those of the refractor already described. The greater weight, however, and the convenience of having the body of the instrument kept as low as possible, owing to the fact of the eye-piece being at the upper end of the tube, have necessitated various modifications in the forms to which these principles are applied. Plates [III.] and [IV.], and Fig. 13, illustrate the altazimuth and equatorial forms of mounting as applied to reflectors of various sizes, Fig. 13 being a representation of Lassell's great 4-foot reflector.
20-inch Reflector, Stanmore Observatory.
And now, having his telescope, whatever its size, principle, or form of mounting, the observer has to proceed to use it. Generally speaking, there is no great difficulty in arriving at the manner of using either a refractor or a reflector, and for either instrument the details of handling must be learned by experience, as nearly all makers have little variations of their own in the form of clamps and slow motions, though the principles in all instruments are the same. With regard to these, the only recommendation that need be made is one of caution in the use of the glass until its ways of working have been gradually found out. With a knowledge of the principles of its construction and a little application of common-sense, there is no part of a telescope mounting which may not be readily understood. Accordingly, what follows must simply take the form of general hints as to matters which every telescopist ought to know, and which are easier learned once and for all at the beginning than by slow experience. These hints are of course the very commonplaces of observation; but it is the commonplace that is the foundation of good work in everything.
If possible, let the telescope be fixed in the open air. Where money is no object, a few pounds will furnish a convenient little telescope-house, with either a rotating or sliding roof, which enables the instrument to be pointed to any quarter of the heavens. Such houses are now much more easily obtained than they once were, and anyone who has tried both ways can testify how much handier it is to have nothing to do but unlock the little observatory, and find the telescope ready for work, than to have to carry a heavy instrument out into the open. Plate [IV.] illustrates such a shelter, which has done duty for more than twelve years, covering an 8½-inch With, whose tube and mounting are almost entirely the work of a local smith; and in the Journal of the British Astronomical Association, vol. xiv., p. 283, Mr. Edwin Holmes gives a simple description of a small observatory which was put up at a cost of about £3, and has proved efficient and durable. The telescope-house has also the advantage of protecting the observer and his instrument from the wind, so that observation may often be carried on on nights which would be quite too windy for work in the open.
