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96 Herschel, in honour of the discoverer, and both these names were used for many years. But still other people said that for anybody to have his name attached to a planet was far too great an honour, whether he were discoverer or king; and ultimately these got their way by establishing the name Uranus.

Herschel's largest mirror was four feet wide; but afterwards Lord Rosse made one six feet wide, which is, up to the present, the largest telescope ever made. A man could stand in the mouth of it. The making of the mirror itself was a great achievement, but that was by no means all; to fit it into a telescope that could be easily handled was a triumph of engineering skill and could only have been accomplished by a great engineer such as Lord Rosse was. One of his sons is also a great engineer; he is the Mr. Parsons who invented the steam turbine, and made a success of it in spite of enormous discouragements; and he told us, you remember, about the diamonds in meteorites.

Herschel's four-foot mirror and Lord Rosse's six-foot are still in existence, and the latter is actually in use in the telescope, though the former has been dismounted. But the surfaces of both have become tarnished, so that they no longer show things clearly. They are made of a peculiar kind of metal called speculum metal, which remains bright for some time, but gradually tarnishes in the course of years in a manner difficult to remedy. To avoid this gradual deterioration, mirrors have been made recently of glass which is then silvered with a thin film on the front surface. This film also tarnishes after