HERSCHEL, WILLIAM. - THE BASIS OF INTERFEROMETRIC IMAGING IN ASTRONOMY.

Experiments for ascertaining how far Telescopes will enable us to determine very small Angles, and to distinguish the real from the spurious Diameters of celestial and terrestrial Objects: with an Application of the Result ofthese Experiments to a series of Observations on the Nature and Magnitude of Mr. Harding's lately discovered Star. Read december 6, 1804.

(London, W. Bulmer and Co., 1805). 4to. No wrappers as extracted from "Philosophical Transactions" 1805 - Part I. Pp. 31-64 and 1 engraved plate. Clean and fine.

First appearance of an importent paper, founding the metric of interstellar space.

It was the contemporous discoveries of the first minor planets, ceres in 1801, Pallas in 1802 and Juno in 1803, that prompted Herschel to investigate the origin of the spurious diameters of stars.
"Were their apparent diameters as real as those of planets or spurious as for stars? To address this question Herschel conducted an extensive series of experiments in his garden in Slough, examining through his telescope small globules of differing sizes and materials placed in a tree some 800 ft (ca. 244 m) away (Herschel 1805). His observations showed that for the smallest globules the diameters were all spurious and all of the same size. Furthermore, he found that, if just the inner part of the aperture of the telescope were used, the spurious diameters, whether of globules or of stars, were larger. If the whole aperture was employed, the diameters were smaller, and if only an outer annular aperture was used the diameters were smaller still. This experimental discovery that unfilled apertures can be used to obtain high angular resolution remains today the essential basis for interferometric imaging in astronomy (in particular Aperture Masking Interferometry). The theoretical justifi­cation of this result came with Airy's analysis of the diffraction pattern of a circular aperture 30 years later (Airy 1835), and it took a further 30 years before the idea of using multiple apertures was developed. In an early study the Reverend W. R. Dawes noted that he had `frequently found great advantage from the use of a perforated whole aperture' and that when observing Venus this produced `a central image of the planet perfectly colourless, and very sharply de­ned' (Dawes 1866). But it was left to Fizeau, in his submission to the Commission for the Prix Bordin the following year, to remark on `une relation remarquable et n´ecessaire entre la dimension des franges et celle de la source lumineuse' and suggest that by using an interferometric combination of light from two separated slits `il deviendra possible d'obtenir quelques donn´ees nouvelles sur les diametres angulaires de ces astres' (Fizeau 1868)."

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