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. [Illustration: PLATE III. 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.