Towards the end of the last century Sir William Herschel invented and constructed the reflecting telescope which bears his name. His great object was to avoid the loss of light consequent on the double reflection which took place in all instruments constructed up to that time, and he succeeded at last in making a telescope in which the observer looked directly through the eye-piece at the image formed by the mirror, which was inclined in such a manner that the rays were reflected to the lower edge of the open end of the tube. In using this kind of telescope the observer is placed with his back to the object he wishes to examine, a position that is even more astonishing to those unaccustomed to the use of a Herschellian telescope than the one assumed when employing an instrument of the Newtonian construction. This position has the defect of causing a small portion of the rays proceeding from the object to be intercepted by the head of the observer, but the amount of light lost is so small in comparison to the size of the mirror that in practice it amounts to nothing. The dimensions of the telescope constructed by Herschel were enormous for that day. It measured 40 feet long, and the mirror was 4 feet in diameter. It was supported by a complicated system of scaffolding, pulleys, and cords, and was capable of magnifying an object 6,000 times. It was by means of this splendid instrument that Sir William Herschel made those wonderful discoveries in astronomy which are inseparably associated with his name. With it he discovered the planet Uranus, many of the double stars, and a large number of nebulæ, which up to that time were unknown. His son, Sir John Herschel, inherits his father’s talents as an astronomer, and has enriched science with numberless observations and discoveries of the greatest importance made with this fine instrument. Fig. 47 shows the construction of the Herschellian telescope, and the path of the rays may be easily followed by the student without any help from us.