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Rh his time at the beginning of the century; and he contributed to the Royal Society on these subjects four papers in rapid succession. He was led to the inquiry by his search for the most suitable dark glasses for solar observation, in the course of which he found that some materials were opaque to light and others to heat. His papers have been called "the first exposition worth mentioning of the principles of radiant heat". In this exposition he showed that radiant heat obeyed the laws of reflection, refraction and dispersion. His investigation of the infra-red heat rays led to one of his greatest discoveries—that of the invisible portions of the solar spectrum.

Early in his career, Herschel paid considerable attention to the Moon. His second paper in 1780 dealt with his measures of the height of the lunar mountains. In 1783 a sensation was caused in scientific circles by the news that Herschel had seen lunar volcanoes in violent eruption. In a letter to Magellan, a Portuguese amateur astronomer, he stated that on 4th May, he "perceived in the dark part of the Moon a luminous spot. It had the appearance of a red star of about the fourth magnitude." In 1787 he communicated a paper to the Royal Society, in which he announced the appearance of other three volcanoes, and in which he promised the Society an account of the eruption of 4th May, 1783. This account was never forthcoming. The leading French astronomers were inclined to the view that the appearances were actually due to earth-shine. It is possible that Lalande, who visited Slough in 1788, may have converted Herschel to this view. At all events, nothing further was published concerning the supposed volcanoes.

Herschel believed the Moon to be both habitable and inhabited. Yet he laid it down in 1794 "that we perceive no large seas in the Moon, that its atmosphere (the existence of which has even been doubted by many) is extremely rare and unfit for the purposes of animal