The advantages that Herschel found in 1783, 1784, and 1785, in employing telescopes of twenty feet and with large apertures, made him wish to construct much larger still. The expense would be considerable; King George III. provided for it. The work, begun about the close of 1785, was finished in August, 1789. This instrument had an iron cylindrical tube, thirty-nine feet four inches English in length, and four feet ten inches in diameter. Such dimensions are enormous compared with those of telescopes made till then. They will appear but small, however, to persons who have heard the report of a pretended ball given in the Slough telescope. The propagators of this popular rumour had confounded the astronomer Herschel with the brewer Meux, and a cylinder in which a man of the smallest stature could scarcely stand upright, with certain wooden vats, as large as a house, in which beer is made and kept in London. Herschel's telescope, forty English feet[18] in length, allowed of the realization of an idea, the advantages of which would not be sufficiently appreciated if I did not here recall to mind some facts. In any telescope, whether refracting or reflecting, there are two principal parts: the part that forms the aerial images of the distant objects, and the small lens by the aid of which these images are enlarged just as if they consisted of radiating matter. When the image is produced by means of a lenticular glass, the place it occupies will be found in the prolongation of the line that extends from the object to the centre of the lens. The astronomer, furnished with an eye-piece, and wishing to examine that image, must necessarily place himself _beyond_ the point where the rays that form it have crossed each other; _beyond_, let us carefully remark, means _farther off_ from the object-glass. The observer's head, his body, cannot then injure the formation or the brightness of the image, however small may be the distance from which we have to study it. But it is no longer thus with the image formed by means of reflection. For the image is now placed between the object and the reflecting mirror; and when the astronomer approaches in order to examine it, he inevitably intercepts, if not the totality, at least a very considerable portion of the luminous rays, which would otherwise have contributed to give it great splendour. It will now be understood, why in optical instruments where the images of distant objects are formed by the reflection of light, it has been necessary to carry the images, by the aid of a second reflection, out of the tube that contains and sustains the principal mirror. When the small mirror, on the surface of which the second reflection is effected, is plane, and inclined at an angle of 45 deg. to the axis of the telescope; when the image is reflected laterally, through an opening made near the edge of the tube and furnished with an eye-piece; when, in a word, the astronomer looks