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the focal point at a distance from the large mirror of rather more than the respective foci added together: the latter reflects the rays

through a hole cut or cast in the centre of the large mirror to the eye-piece. The inner lens of the eye-piece hastens the fays to a focal point before they reach the front lens, the rays therefore cross a second time, thereby insuring an erect picture of objects viewed.

Some few years ago the Gregorian telescope was commonly met with, but it has lately been superseded by the other forms. .Its principal advantage for terrestrial purposes is the erect image.

The speculum is generally fixed in position, but adjustments are provided to the small mirror and eye-piece. To adjust these, first, unscrew the eye-piece and remove its lenses. In front of the eye-piece is a disc with a small hole, unloose the two screws which hold the disc in position and set the small hole central with the surrounding metal. Having clamped it there, look through the hole and adjust the small speculum by means of the small screws working against it until a complete picture of the large speculum is seen in its centre. Fig. 15. Now replace the lenses and notice if the field of view is equally illuminated. If it is so, all is sufficiently correct for general observation. But if the field appears brighter in one part, slightly alter the position of the eye-hole by shifting the little disc.

The focussing of a Gregorian telescope is generally effected by means of a long screw which when revolved alters the distance between the mirrors.

THE CASSEGRAINIAN TELESCOPE.

Fig. 7-

In the ordinary form of this instrument, the rays of light from the speculum fall upon a convex small mirror before they come to a focus, and from the convex mirror are returned through the centre of the large mirror to the eye-piece. It is difficult to say why this form has been so seldom made, as it possesses several important advantages, one of the chief, being that a Cassegrainian mirror can be

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