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He considered that, while the superiority of the heliometer had been proved, the results would be still better with the points of light shown by minor planets rather than with the disc of Mars. In 1888-9, at the Cape, he observed the minor planets Iris, Victoria, and Sappho, and secured the co-operation of four other heliometers.
Among other valuable contributions to this difficult branch may be instanced Bruno Peter's measurements of eleven stars with the Leipzig heliometer, 1887-92; Kapteyn's application of the method by differences in right ascension to fifteen stars observed on the meridian 1885-89; and Flint's more recent similar determinations at Madison, Wisconsin.
But this represents only the angle subtended by the star's disk. To learn its linear diameter, we must know its distance. Four determinations of the parallax, which determines the distance, have been made. Elkin, with the Yale heliometer, obtained 0.032 of a second of arc. Schlesinger, from photographs taken with the 30-inch Allegheny refractor, derived 0.016.
Among these are an anemometer slowly revolving in the light air, maximum and minimum bulbs in the shade, on the ground and beneath it, a most ingenious sun dial, and a heliometer. Walking inland along the central avenue, we pass some native shops, one of which bears the interesting name of Williams Brothers.
It is true that a daguerreotype, taken by Berkowski with the Königsberg heliometer during the eclipse of 1851, is still valuable as a record of the corona of that year; and some subsequent attempts were made to register partial phases of solar occultation, notably by Professor Bartlett at West Point in 1854; but the ground remained practically unbroken until 1860.
Such catalogues vary in their degree of accuracy; and fundamental catalogues of standard stars have been compiled. These require extension, because the differential methods of the heliometer and the camera cannot otherwise be made absolute.
Both astronomers agreed to use the "differential" method, for which their instruments and the vicinity to their selected stars of minute, physically detached companions offered special facilities. He then had his heliometer taken down and repaired, after which he resumed the inquiry, and finally terminated a series of 402 measures in March 1840.
From measurements of 200 negatives taken in 1886, he derived for that classic star a parallax of 0·438", in satisfactory agreement with Ball's of 0·468". A detailed examination convinced the Astronomer-Royal of its superior accuracy to Bessel's result with the heliometer.
The heliometer parallax is doubtless less reliable than the photographic ones, and Doctor Adams states that the spectral type and luminosity of Betelgeuse make his value less certain than in the case of most other stars.
But there is one instrument of great value, the heliometer, which is not used at Greenwich. This serves the purpose of a double image micrometer, and is made by dividing the object-glass of a telescope along a diameter. Each half is mounted so as to slide a distance of several inches each way on an arc whose centre is the focus. The amount of the movement can be accurately read.
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