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be as long as possible. It sometimes happens that a mirror is accidentally worked very slightly cylindrical, and if so, it may be comparatively useless for a Newtonian telescope, but perhaps exactly suitable for a Herschelian, as it will show the best image of a star when the optical axis is not exactly at right angles to the surface of the mirror; the nearer to the side of the tube the best image is formed the better the performance will be. The adjustments of a Herschelian telescope are very simple. Remove the eye-piece and adjust the screws, which act on the cell of the mirror, until the entire surface of the latter is seen illuminated by the sky end of the tube, Fig. 15 (without the black centre spot). Turn the telescope on a star, and, if necessary, alter the direction of the eye tube till a star presents nearly a circular disc inside and outside the focal point. If the mirror is slightly cylindrical, first adjust as far as possible, and finally revolve the mirror in its cell, a few degrees at a time, until a circular disc is obtained.

THE NEWTONIAN TELESCOPE.

A large majority of reflecting telescopes now used are of the Newtonian form, and therefore careful study of the principles and adjustments of it are required, in order that the amateur may not only be able to rectify any accidental adjustment, but also may perfectly grasp the principle on which it is made. A large amount of such knowledge can be applied to the adjustments of all other forms of reflecting telescopes.

Principles of the Newtonian Reflector.—The parallel rays of light from a star or other celestial object, passing down the open telescope tube, are received upon a concave parabolic speculum, and returned as convergent towards a point just outside the end of the tube, and also in the centre or axis of it. The rays of light are not allowed to focus without the tube, but are intercepted and bent, at right angles, to an eye-piece placed in the side of the body-tube, by a small flat mirror, placed at an angle of 450. This small mirror is in shape an oval, but appears circular when viewed either from the eye-piece, or from the centre of the mirror, were it possible.

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