Part 8 (2/2)

[30] BODE'S _Jahrbuch_, 1823, p 222

CHAPTER IV

REVIEW OF THE SCIENTIFIC LABORS OF WILLIAM HERSCHEL

In this chapter I shall endeavor to give such explanations as will enable the general reader to follow the course of discovery in each branch of astronoh the period of HERSCHEL'S life, and up to the state in which he left it

A more detailed and precise account, which should appeal directly to the professional astronomer, will not be needed, since ARAGO has already fulfilled this want in his ”_analyse de la vie et des travaux de Sir WILLIAM HERSCHEL_,” published in 1842 The few misconceptions there contained will be easily corrected by those to whom alone they are of consequence The latter class of readers may also consult the abstracts of HERSCHEL'S iven in ”_A Subject-index and a Synopsis of the Scientific Writings of Sir WILLIAM HERSCHEL_,” prepared by Dr HASTINGS and myself, and published by the Smithsonian Institution

An accurate sketch of the state of astronoland and on the Continent, in the years 1780-1820, need not be given It will be enough if we remember that of the chief observatories of Europe, public and private, no one was actively devoted to such labors as were undertaken by HERSCHEL at the very beginning of his career

His observations on variable stars, indeed, were in the same line as those of PIGOTT; FLAUGERGUES and DARQUIER, in France, had perhaps preceded him in minute scrutiny of the sun's surface, etc; but, even in that department of observation, he at once put an immense distance between himself and others by the rapid and extraordinary advances in the size and in the excellence of his telescopes Before his tiorian and Newtonian telescopes of SHORT, and the soes before, how his patient zeal had succeeded in i upon these There was no delay, and no rest Steadily the art of ed forward, until he had finally in his hands the forty-foot telescope

It must be admitted that this was the limit to which the eneration The optical and mechanical difficulties which prevented a farther advance required time for their solution; and, indeed, some of these difficulties are scarcely solved at this day It er than three feet in aperture has yet realized our expectations

_The Improvement of Telescopes and Optical Apparatus_

It will be of interest to give in this place soe forty-foot reflector, of four feet aperture, made by HERSCHEL

Its history extends from 1785 to 1811 Its reatest triuenious in all its parts, as may be seen from the elaborate description and plates of it published in the _Philosophical Transactions_ for 1795 One of its ood definition, for, by means of it, the two small satellites of _Saturn_ (_Mimas_ and _Enceladus_) were discovered, and these discoveries alone would make it faeneral than it absolutely performed Its merits were after a while decried, and HERSCHEL even felt obliged to state why he did not always employ it in his observations His reasons were perfectly valid, and such as any one e aorder was a serious tax; it required more assistants than his twenty-foot telescope, and he says, ”I have er telescope when a smaller will answer the purpose”

It still rereat optical and e reflectors of Lord ROSSE, some sixty years later, and several of the forty-foot telescopes of the present day even have done less useful work Its great feat, however, was to have added two satellites to the solar syste is taken:

”When I resided at Bath I had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences This I acquired by degrees at that place, where in my leisure hours, by way of amusement, I made several two-foot, five-foot, seven-foot, ten-foot, and twenty-foot Newtonian telescopes, beside others, of the Gregorian forhteen inches, and two, three, five, and ten feet focal length In this way I made not less than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot orian telescopes[32]

”The number of stands I invented for these telescopes it would not be easy to assignIn 1781 I began to construct a thirty-foot aerial reflector, and having made a stand for it, I cast the mirror thirty-six inches in dia I cast it a second time, and the furnace I had built in ian planet was discovered, and this interrupted the work for a tiood twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope After two years' observation with it, the great advantage of such apertures appeared so clearly tothe now sufficiently provided with experience in the hich I wished to undertake, the President of the Royal Society, who is always ready to pron before the king His Majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal bounty

”In consequence of this arrangean to construct the forty-foot telescope about the latter end of 1785[33] The ork of the stand andthe required motions to the instrument were immediately put in hand In the whole of the apparatus none but cos of every part of it, by which it was easy to execute the work, as I constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the sa, I also began the construction of the great , and polishi+ng, and the as in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials froh

”Here, soon after rees the whole structure was raised as it now stands, and the speculuhly polished and put into the tube, I had the first view through it on February 19, 1787 I do not, however, date the co of the instruement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not periven to it

”A secondFebruary 16 we recast it, and it proved to be of a proper degree of strength October 24 it was brought to a pretty good figure and polish, and I observed the planet _Saturn_ with it But not being satisfied, I continued to work upon it till August 27, 1789, when it was tried upon the fixed stars, and I found it to give a pretty sharp iht, owing to ust the 28th, 1789, having brought the telescope to the parallel of _Saturn_, I discovered a _sixth_ satellite of that planet, and also saw the spots upon _Saturn_ better than I had ever seen the of the forty-foot telescope from that time”

Another satellite of _Saturn_ was discovered with the forty-foot on the 17th of September (1789) It was used for various observations so late as 1811 On January 19, of that year, HERSCHEL observed the nebula of _Orion_ with it This was one of his last observations

The final disposition of the telescope is told in the following extract from a letter of Sir JOHN HERSCHEL'S to Mr WELD, Secretary of the Royal Society:

”COLLINGWOOD, _March 13, 1847_

”In reply to your queries, respecting the forty-foot reflecting telescope constructed byGEORGE III munificently defrayed the _entire_ cost of that instru, of course, all preparatory cost in the nature of construction of tools, and of the apparatus for casting, grinding, and figuring the reflectors, of which tere constructed), at a total cost of 4,000 The ork of the telescope being so far decayed as to be dangerous, in the year 1839 I pulled it down, and piers were erected on which the tube was placed, _that_ being of iron and so well preserved, that, although not more than one-twentieth of an inch thick, when in the horizontal position it sustained within it all my family, and continues to sustain inclosed within it, to this day, not only the heavier of the two reflectors, but also all the more important portions of the machinery