Page:The wonders of optics (1869).djvu/168

160 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.



The rays, proceeding from the object at which the instrument is pointed, are first reflected from the surface of the principal mirror 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. 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 tele