one end of the tube. At a distance from the larger mirror, less than its focal length, is placed at an angle of 45° to the optic axis of the telescope, a plane reflector, by which the rays proceeding from the object are turned to the side of the larger tube, where there is a smaller one in which the eye-piece for viewing them is placed. Instead of the plane reflector, Foucault uses a prism. In all these telescopes the central rays are lost, because in the Gregorian and Cassegrainian arrangements the central portion of the mirror is cut away, and in the Newtonian the central rays are intercepted by the plane mirror. Fig. 183. THE HERSCHELIAN TELESCOPE. In the Herschelian Telescope the large speculum is not fixed in the tube with its diameter at right angles to the axis of the tube, but is slightly inclined thereto; by this means the image of the object observed is brought to the interior edge of the tube, where it is directly examined by the eye-piece, instead of through the medium of the 2nd reflector. The advantages of this plan are, that not only can observations be made with greater ease, but a large saving of light is effected by dispensing with the 2nd reflector. It was with an instrument constructed in this way that Sir W. Herschel made those numerous and important observations and discoveries many of which have already been brought under the notice of the reader. A modification of Newton's arrangement has been made use of with satisfactory results by Mr. James Nasmyth, the eminent machinist. The rays reflected from the great speculum are received either upon a small speculum, or upon a prism placed in the axis of the tube, between the focus and the great speculum. By this they are reflected at right angles, and the image is formed in one of the trunnions (made tubular for the purpose) on which the instrument turns. The image is then viewed in the usual way. The advantage accruing from this arrangement is that the great tube can be moved through any extent of altitude, whilst the lateral or trunnion tube, in which is placed the eye-piece, remains in one position, and thus the observer can survey any vertical circles in the sky without continually changing his station. The inventor has a reflector of this construction erected at his residence at Penshurst, the tube of which is 28 feet long and 4 feet 6 inches. in diameter. The azimuthal movement is given by a turn-table similar to that used on railways for turning locomotive engines. Reflecting telescopes for many years past have been but little used (always excepting a few large ones of what may be termed historic note), but their employment is now on the increase; and there are those who believe that the more modern sort, having mirrors made of silvered glass instead of speculum metal, will soon be in a position to compete with refractors, in regard to efficiency balanced against lesser cost. At any rate, attention has been a good deal drawn to them of late, and With of Hereford, Foucault of Paris, and Steinheil of Munich, have turned out instruments which have been very favourably reported upon by competent judges. With confines his attention to mirrors, and leaves the mounting to be done by others, in particular by Mr. J. Browning. Fig. 184 is a representation of the bed employed by Browning for With's mirrors. The bottom of the mirror A is ground to an approximately true surface, and the same thing is done with the bottom of the inner cell B, on which it rests. Parallelism is obtained by the use of the adjusting-screws D D and E E; and the mirror can be removed |