The text below was published as part of the 1856 Report of the Astronomer Royal
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ADDRESS TO THE INDIVIDUAL MEMBERS OF THE BOARD OF VISITORS OF THE ROYAL OBSERVATORY GREENWICH BY THE ASTRONOMER ROYAL.
THE present time is peculiarly favourable for a decision of the Visitors on an important part of the instrumental equipment of the Royal Observatory; namely, on the kind of Equatoreal with which it ought to he furnished. The matter will, perhaps, be advantageously submitted to their notice, if I commence with an account of the Equatoreal establishment as it has existed for some years past, and of its condition at the present time.
In the year 1835 (when my official connexion with the Observatory commenced), and for several years before that time, the Equatoreal apparatus of the Observatory was contemptible. A small telescope, mounted on a rude polar axis, occupied the North-west Dome; and Shuckburgh's Equatoreal, carrying a telescope with a defective object-glass of 4-inch aperture, was in the North-east Dome (now called the North Dome), in the place which it still holds. The frame of' this instrument is very weak, but it is furnished with very large graduated circles, both for hour-ankle and for polar distance. The positions of these two instruments are so strangely misadapted to their purpose, that I am utterly at a loss to conceive under what circumstances their places were selected. The Octagon Room towers over them in such a manner that nearly the whole sky from south to east, to the height of 40º or more, is hidden from the North-west Dome; and nearly the whole slay from south to west, to the height of more than 50º in some parts, is hidden from the North-east Dome.
In the year 1837, my valued friend Mr. Sheepshanks presented the Observatory with an object-glass of 6¾- inches aperture. There was then vacant a shall dome (now called the East Dome), with a view uninterrupted by any near buildings, whose dimensions were exactly adapted to receive this telescope, and in the center of which there stood a pier on which a mounting, resembling generally the German mount might be placed. In this position, and with this form of mounting, the Sheepshanks telescope was accordingly placed. It has been a valuable addition to the instruments of the Observatory; but it is subject to two defects. First, the object-glass, though very good (especially for sidereal purposes) when used with care, is somewhat irrational, as well as veiny, and is not such as ought to be exhibited to a foreigner as the best telescope for general purposes in the National Observatory. Secondly, the mounting, which necessarily carries very small circles, makes it unfit for accurate determination of the place of a celestial body.
The determinations of place, rather than the observations requiring no graduated circles, must, I think, always be considered as the proper object for instruments in this Observatory. We are compelled, therefore, to rely on the Shuckburgh Equatoreal for what I consider to be, here, the normal applications of an Equatorial.
Within the last two years the Shuckburgh Equatoreal has come to a condition that imperatively requires attention. The divisions of the hour-circle are illegible. Its construction renders it difficult to take to pieces for redivision, and I can hardly hope that it could be put together again with due firmness. New microscopes are required. The estimate for reinstating it is under £100; but I fear that this sum would be much exceeded before the instrument was replaced. And when this was done, we should have a small indifferent telescope, on a weak frame, in a position bad beyond anything that an astronomer ever imagined.
I think now that I may well submit to the consideration of the Visitors, whether it would not be preferable at once to seek a better telescope, and to endeavour to mount it upon a better frame in a better position.
It is known to the Visitors that I have uniformly objected to any luxury of extra-meridional apparatus, which would materially divert us from a steady adherence to the meridional system which both reason and tradition have engrafted on this Observatory; but I feel that our present instruments are insufficient even for my wishes; and I cannot overlook the consideration that due provision must be made for future interests, and that we are nearer by twenty years to the time when another judgment must decide on the direction which shall be given to the force of the Observatory.
I have endeavoured to obtain information on object-glasses free for sale at this time. With one exception, I cannot find any. The Munich Artists profess themselves willing to make one of any aperture from 6 to 13 or 14 French inches, but in not less than from one to two years. Mr. Merz, however, has one object-glass of 12 French inches aperture, and 17½ French feet in focal length, at the price of 13,300 florins (about £1,100), and this, perhaps, is still free for purchase. I am prepared to indicate a position (at the south-east angle of the new Record Room), where an Equatoreal of this size may be advantageously mounted.
It is necessary, in even suggesting the possibility of procuring such an instrument, to enter into some details as to the form of mounting. Viewing (as I do) the determination of celestial places as being, in this Observatory, paramount to all others, I at once give up the German form of mounting as insufficient. I confine my attention to the English form, with double polar cheeks; and I think that the question lies principally between a braced frame of wood (such as that of the Northumberland Equatoreal at Cambridge), and a frame of metal plates (as that of the Liverpool Equatoreal). I prefer the frame of the Northumberland Equatoreal, slightly modified, as equally firm, and cheaper and more convenient.
But, in any case, the expense must be considerable. I could scarcely hope that the building and mounting could be provided for £2,000 in addition to the price of the object-glass.
It appears to me that the question is now in a state in which it would be wrong in me to take any further step, or to express any further opinion, without consulting the Visitors. And having laid before them all the ideas that occur to me as bearing in an important degree on this matter, I solicit the early expression of their opinion, so as to enable me (if such should be the direction of their decision) to act promptly, both with our own Government and with the artists who would furnish the lenses and mountings.
Royal Observatory, Greenwich,
G. B. AIRY.
1855, October 18.