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Updated: June 3, 2025
Lassell's plan was a totally different one; he employed the crossed axes of the true equatoreal, and his success removed, to a great extent, the fatal objection of inconvenience in use, until then unanswerably urged against reflectors.
It would be out of place to describe here the practical adjuncts of a modern equatoreal the adjustments for pointing it, the clock for driving it, the position-micrometer and various eye-pieces, the photographic and spectroscopic attachments, the revolving domes, observing seats, and rising floors and different forms of mounting, the siderostats and coelostats, and other convenient adjuncts, besides the registering chronograph and numerous facilities for aiding observation.
This beautiful method was first realised by Professor Hale in June, 1891. A "spectroheliograph," consisting of a spectroscopic and a photographic apparatus of special type, attached to the eye-end of an equatoreal twelve inches in aperture, was erected at Kenwood in March, 1891; and with its aid, Professor Hale entered upon original researches of high promise for the advancement of solar physics.
Sir Howard Grubb finished in 1893 a 28-inch achromatic for the Royal Observatory, Greenwich; the Thompson equatoreal, mounted at the same establishment in 1897, carries on a single axis a 26-inch photographic refractor and a 30-inch silvered-glass reflector; the Victoria telescope, inaugurated at the Cape in 1901, comprises a powerful spectrographic apparatus, together with a chemically corrected 24-inch refractor, the whole being the munificent gift of Mr.
What were taken to be the polar regions appeared comparatively dusky. Similar observations were made at Nice by MM. Perrotin and Thollon, March to June, 1884, a lucid spot near the equator, in addition, indicating rotation in a period of about ten hours. The discrepancy was, however, considerably reduced by Perrotin's study of the planet in 1889 with the new 30-inch equatoreal.
Then there was the sensation of Clark's 36-inch for the Lick Observatory in California, and finally his tour de force, the Yerkes 40-inch refractor, for Chicago. At Greenwich there is the 28-inch photographic refractor, and the Thompson equatoreal by Grubb, carrying both the 26-inch photographic refractor and the 30-inch reflector. At the Cape of Good Hope we find Mr.
Its resumption, after some years, was rendered possible by M. Bischoffsheim's gift of 25,000 francs for expenses, and the coudé or "bent" equatoreal has been, since 1882, one of the leading instruments at the Paris establishment. Its principle is briefly this: The telescope is, as it were, its own polar axis.
He was followed by Dominique Cassini, who detected bands like Jupiter's belts. Herschel established the rotation of the planet in 1775-94. From observations during one hundred rotations he found the period to be 10h. 16m. 0s., 44. Herschel also measured the ratio of the polar to the equatoreal diameter as 10:11.
As the image of the sun passes over the first slit the photographic plate is moved at the same rate and in the same direction behind the second slit; and as successive sections of the sun's image in the equatoreal enter the apparatus, so are these sections successively thrown in their proper place on the photographic plate, always in K light.
Sir George Airy's incredulity vanished in the face of the striking coincidence between the position assigned by Leverrier to the unknown planet in June, and that laid down by Adams in the previous October; and on the 9th of July he wrote to Professor Challis, director of the Cambridge Observatory, recommending a search with the great Northumberland equatoreal.
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