Rosse Telescope
The Encyclopedia of Founding (1892)
This wonderful telescope was made by the renowned astronomer Lord Rosse (born 1800, died 1867), who devoted a great portion of his life to the improvement of reflecting telescopes, and succeeded in mounting one of 3-feet aperture at his home, Birr Castle, Ireland, in the year 1839. In 1842 the now celebrated six-foot reflector was successfully cast and polished, being finally mounted in 1845. The immense tube which contains it is 54 feet long and 7 feet diameter. The speculum metal employed for casting this reflector consisted of 4 equivalents of copper to 1 of tin, which is equal in weight to the following proportions: Copper 252.8, tin 117.8. This alloy is exceedingly hard and brittle, will take a beautiful white polish, and does not readily tarnish; but, owing to its extreme brittleness, there was much difficulty experienced in obtaining a speculum casting of this magnitude absolutely free from shrinkage cracks, gas-holes, and a decided tendency to warp out of shape. To obviate these difficulties, the cooling of the mass must be controlled and the gas eliminated; all of which, we are told, was successfully accomplished by Lord Rosse after a somewhat novel fashion. He formed the face side of his mould with hoop-irons, side by side, and edge up. When this iron bed had been thus made, the outside edge was formed with sand, and the casting poured as an open mould. The closely packed hoop-iron bed contained comparatively no gas-producing substances, as sand does, and whatever gas might exude from the metal thereon would be instantly pressed through the countless interstices by the superincumbent pressure of the metal above. See SPECULUM METAL; TIN. Rosthorn's Austrian Metal for Cannon.--See GUN-METAL.
What Has Changed in the Last Century
When this was written in 1892 the six-foot "Leviathan of Parsonstown" was still the largest telescope ever built, as it had been since 1845. It was little used after Lord Rosse's death and the tube stood empty for most of the twentieth century, the speculum having been removed (one of the original discs is in the Science Museum, London); the mounting and tube were restored at Birr Castle in the 1990s with a new mirror, and the instrument can again be pointed at the sky as a working exhibit. Its record aperture was surpassed by the 100-inch Hooker reflector at Mount Wilson in 1917, and today's largest optical telescopes have mirrors of 10 metres and more, with the 39-metre Extremely Large Telescope under construction in Chile. The brittle copper-tin speculum described here has vanished from telescope-making entirely: modern mirrors are thin glass or ceramic blanks coated with a microscopically thin film of aluminium. Rosse's most lasting result, the spiral form he sketched in M51 in 1845, was explained only in the 1920s when such "nebulae" were shown to be separate galaxies.
Written for this edition, 2026.