M. Cassegrain substituted for the concave mirror a convex one, and placed it within the focus, thereby shortening the telescope by twice the focal length of the small mirror. The Cassegrainian is therefore the most compact form of reflecting telescope. Both the Gregorian and Cassegrainian give fair definition with spherical mirrors, and are therefore easy instruments to manufacture. They are generally focussed by shifting the position of the small mirror.

Sir I. Newton was struck with the difficulty of viewing objects near the zenith, and therefore devised the telescope which bears his name, and in which the small mirror is flat, and being placed at an angle of 450, reflects the rays at right angles to an eye-piece placed in the side of the tube.

Sir W. Herschel discarded the small mirror, and by tilting his large speculum, formed the image close by the side of the tube, where it could be viewed by the eye-piece.

The principal defect of reflecting telescopes is that they cannot be used as instruments of precision, for instance, as part of a transit-circle. The cause of this is that a mirror does not admit of being so firmly held as an object glass. The less strain there is on a mirror, the better it will perform. It will therefore sometimes happen that an object will not be found exactly in the centre of the field, however correctly the circles of an equatorially-mounted reflector may be set. Unless it be desired to ascertain the exact position of a celestial object by the circle readings, a slight error is of no consequence, as the object is certain to be in the field of a moderately high power with careful setting.

To a certain extent an acknowledged superiority of refractors over reflectors is the greater light-giving power of the former, aperture for aperture. An amount of light, very variously estimated by different observers, is lost by the several reflections; as the aperture of the telescope is increased this disparity diminishes, the larger object-glasses, being of necessity thicker, absorb a greater amount of light, whereas the light from a mirror has no absorbing medium to diminish its intensity. It may therefore be confidently asserted that the lightgiving power of a very large reflector may equal if not exceed that of an object glass of equal aperture. While, however, a smaller object-glass is equal to a larger reflector in the above respect, it will be inferior in definition and penetrating power. An example of this is seen in the beautiful definition given by an unsilvered glass mirror on bright objects, as the Moon and Venus.

An important defect attributed to reflectors is their unsteady definition. The end of the telescope tube being open, air currents are often very troublesome, especially under certain atmospheric conditions. It used to be often necessary to leave the telescope for nearly half-an hour before the best definition was obtained. The effect of

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