ONE of the lesser known tales of Edgar Allen Poe, written in the 1840s, is called The Adventures of One Hans Pfaall, and concerns a Dutchman who ascends to the moon in a balloon. In an apologia appended to the story, the author explains its scientific back ground, and in the course of it he writes:
"The Earl of Ross (sic) telescope, lately constructed in England, has a speculum with a reflecting surface of 4071 square inches 6 ft in diameter, weighing 3 tons and with a focal length of over 50 ft."
The telescope, of course, is not and never was in England. It was built at Birr Castle by William Parsons, Third Earl of Rosse, who was born 196 years ago today on June 17th, 1800.
In his early year Parsons took an active part in politics, but as it has been quaintly put, "the charms of science gradually weaned him away from all pursuits that interfered with its cultivation", and thus it was that he built what was to be, for over 70 years, the largest astronomical telescope in the world.
Apart from its size, the Earl's telescope was unusual in being a reflecting telescope at a time when such instruments were thought to be inferior to the more traditional "refractors". After much experimentation, Rosse concluded that an alloy of copper and tin, mixed in proportions of 2 to 1, was the most effective material for the reflecting surface.
After five attempts and four disasters in five years, the mirror of "the Leviathan of Parsonstown" was successfully cast on April 13th, 1842, in the special foundry set up for the purpose in the moat of Birr Castle. It weighed 4.5 tons (pace Mr Poe), and was an unprecedented 72 inches in diameter.
The great 6-ft speculum mirror was finished in a steam driven grinding and polishing machine, also built to the earl's own design. It took two more years to construct the tube of the telescope, which was then elevated in the gap between two walls over 50 feet high.
With such an enormous aperture, the instrument was able to gather more light than any in existence, and thus able to look further into space than had ever been possible before. It was brought into use in February 1845, and by April Rosse had made his most momentous discovery - that many of the galaxies had a spiral structure.
Only the Great Famine, during which Rosse directed his energies to relief work and gave the major part of his rents to alleviate the poverty of his tenants, was allowed to interfere with these endeavours. Rosse continued an active and productive astronomer until his death in Monkstown, Co Dublin in October 1867.