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Sir William Herschel propounded an explanation of Wilson's observation which received much notice, but which, out of respect for his memory, is not now described, as it violated the elementary laws of heat. Sir John Herschel noticed that the spots are mostly confined to two zones extending to about 35 degrees on each side of the equator, and that a zone of equatoreal calms is free from spots.
At the opposition of 1884, indeed, the distance of the planet was too great to permit of the detection of both elsewhere than at Washington. But the Lick equatoreal showed them, July 18, 1888, when their brightness was only 0·12 its amount at the time of their discovery; so that they can now be followed for a considerable time before and after the least favourable oppositions.
The first plan is that of the "transit instrument," the second that of the "equatoreal." Both were, by a remarkable coincidence, introduced about 1690 by Olaus Römer, the brilliant Danish astronomer who first measured the velocity of light. The uses of each are entirely different.
After only eight days of visibility, it had faded so much as no longer to strike, though still discoverable by the unaided eye; and on February 20 it was invisible with the great Cordoba equatoreal pointed to its known place. But the most astonishing circumstance connected with this body is the identity of its path with that of its predecessor in 1843. This is undeniable. Dr. Gould, Mr.
The reappearance of this strange appurtenance on three subsequent plates left no doubt of its real existence, visually attested at Pulkowa, February 5, 1886, by one of the first observations made with the 30-inch equatoreal. Much smaller apertures, however, sufficed to disclose the "Maia nebula," once it was known to be there.
It is possible that the monumental domes and huge movable tubes of our present observatories will, in a few decades, be as much things of the past as Huygens's "aerial" telescopes. It is certain that the thin edge of the wedge of innovation has been driven into the old plan of equatoreal mounting.
Important series of double-star observations were made by Perrotin at Nice in 1883-4; by Hall, with the 26-inch Washington equatoreal, 1874 to 1891; by Schiaparelli from 1875 onward; by Glasenapp, O. Stone, Leavenworth, Seabroke, and many besides. Finally, Professor Hussey's revision of the Pulkowa Catalogue is a work of the teres atque rotundus kind, which leaves little or nothing to be desired.
He realised the superiority of a circle with graduations over a much larger quadrant. The collimation error was found by reversing the instrument and using a terrestrial mark, the azimuth error by star observations. The time was expressed in fractions of a second. He also constructed a telescope with equatoreal mounting, to follow a star by one axial motion.
The result was a Newtonian of exquisite definition, with an aperture of two, and a focal length of twenty feet, provided by a novel artifice with the equatoreal mounting, previously regarded as available only for refractors. This beautiful instrument afforded to its maker, October 10, 1846, a cursory view of a Neptunian attendant.
The "Great September Comet" of 1882 was in no hurry to withdraw itself from curious terrestrial scrutiny. It was discerned with the naked eye at Cordoba as late as March 7, 1883, and still showed in the field of the great equatoreal on June 1 as an "excessively faint whiteness."
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