THE mirrors in astronomical reflecting telescopes were formerly made of speculum metala hard alloy of copper and tin capable of taking a fine polish and having a fairly high reflecting power. Speculum metal mirrors have been entirely replaced by glass mirrors, coated on the figured surface with a thin film of silver. The silvered surface, when fresh, has a high reflecting power'for wave-lengths greater than 0.375µ at this wave-length the reflecting power is 0-80; at 0.400µ it is 0.85; at 0.450µ it is 0.90 and at 0.700µit is 0.95. On the short wave-length side of 0.375µ the reflecting power falls off rapidly, owing to selective absorption, to a minimum value at about 0.315µ of only 0 .04. This is a serious disadvantage when observations are required in the ultra-violet region. Thus, for example, at a wave-length of about 0.325µthe atmospheric transmission is 0.50 but the reflecting power of silver is only 0.12. In a reflecting telescope, the image is normally produced by reflection at two silvered mirrors, so that the loss by the selective absorption of the silver is much greater than the above figures indicate. These figures refer to a freshly deposited silver film. But the reflecting power steadily falls owing to gradual oxidation or to tarnishing due to the action of sulphur dioxide in the atmosphere. Where such atmospheric contamination is prevalent the reflecting power falls rapidly. At even the most favourable sites, however, there is a gradual fall in reflecting power, so that the mirrors must be periodically dismounted and resilvered. The silver film is easily deposited chemically and though the silvering process itself is not difficult, any satisfactory method of making it unnecessary would be welcomed, for it would imply that the decrease in reflecting power?which necessitates lengthened exposures and is particularly troublesome in some photometric work?had been eliminated. Coating the silver film with a thin film of colourless lacquer has been tried, but the optical perfection of figure is almost inevitably impaired.
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Biographical Memoirs of Fellows of the Royal Society (1961)