Hitherto we have only spoken of refracting telescopes or those instruments provided with a convex object-glass, to collect and refract the rays of light given off by the object we are desirous of examining; but there is another and very important class of instruments, in which the object-glass is replaced by a reflecting mirror. The first reflecting telescope was invented by Dr. Gregory, an English philosopher, about 1650. It consisted of a brass tube, at the lower extremity of which was fixed a concave mirror made of metal, and provided with a hole in its centre for the insertion of the small tube containing the eye-glass. Towards the other end of the telescope was placed a second and smaller mirror, which reflected the image formed by the large mirror, through the eye-piece to the eye. The following figure will show the path of the rays in the Gregorian telescope. [Illustration: FIG. 44.—Section of the Gregorian Telescope. ] The rays A B, proceeding from the object at which the instrument is pointed, are first reflected from the surface of the principal mirror M M on to the small mirror _m_, whence they proceed to form a magnified image at _a b_, which is then again enlarged by the eye-piece appearing to the eye as if placed at A´ B´. The focus in the Gregorian is altered, not by sliding the eye-piece backwards and forwards but by moving the mirror _m_, which is provided with a long screw, to which is attached a handle. At first sight a reflecting telescope has the appearance of a very stumpy-looking refracting instrument, but one instant’s examination will show the observer that the usual object-glass is absent at the end of the tube. In fig. 45 we have a Gregorian telescope, mounted on a tripod stand. [Illustration: FIG. 45.—Gregorian Telescope. ] Whilst experimenting on the Gregorian telescope, Newton made certain improvements in its construction, which we shall proceed to describe. A glance at fig. 46 will show that the path of the rays is much more simple than in the instrument we have just noticed. [Illustration: FIG. 46.—Section of a Newtonian Telescope. ] The rays of light A B are first reflected from the concave mirror M on to the surface of the small plane mirror _m_, which is placed at an angle of 45°, and reflects them as far as the point A´ B´, where they form the image to be magnified by the eye-glass. It is therefore at the _side_ of the instrument, and not at the end, as hitherto, that the observer is placed, and at right angles to the path of the rays. Observers looking at an object through a Newtonian telescope for the first time are generally sufficiently astonished to find that there is really no difficulty after all in seeing round a corner. We shall presently return to the subject of Newtonian telescopes, which were abandoned by astronomers for many years, until they were brought into use again by M. Foucault, a distinguished French philosopher.