TELESCOPE. 



twn is taken with tlie face lool:iiig at the fpcculum, or by 

 what the autlior has called, by way of diftiiiflion, \.\\c front 

 •vi<'-:u, the back "xiiig always turned to the objoCt to be 

 viewed. Befide-v '-lie puUics of elevation, and of a/.imuthal 

 motion, there are others for the purpofe of communication, 

 as well as fpc.il'nnj-pipes, repeating-bells, and iignals by 

 clock-work, which cannot be clearly comprehended without 

 infpeftion, or numerous drawings to be referred to ; but the 

 dexterity of the obferver has rendered fome of thefe fupcr- 

 fluous. The large fpeculuin is enclofed in a llrong iron 

 ring, braced acrois with bars of iron, and an encloiurc of 

 iron and tin (heets makes a cafe for it ; it is lifted by three 

 handles of iron attached to the fides of the ring, and is put 

 into and taken out of its proper place by the help of a move- 

 able crane, running on a carriage, which operation of coiirfe 

 requires great care. Three fmall vanes attached to the edge 

 of the tube at the mouth, afiift to put tlie line of coUiraatioa 

 right, wlicn they are feen refleAed from thefpeculum to the 

 eye-piece. We viCted Slough lately, with a view of ex- 

 amining all the minutiae of the llupendous apparatus tliat is 

 rendered neceifary for the management of this huge telefcope, 

 and that can only be well defcribed on the fpot, and found 

 the fubftance of a letter written by the late Mr. Smcaton on 

 this fubjeft, immediately after a vifit for the exprefs purpofe 

 of infpe<fting]the apparatus then in exiilence, fo accurate, that 

 we avail ourfelves of this fource of information ; and as the 

 letter which is before us is a copy taken from the writer's 

 own manufcript, we have no doubt of its authenticity. It 

 relates however principaily to the twenty-feet inftrument. 



Gray's Inn, Nov. 4th, 1785. 

 " My dear friend, 

 *' Since my laft, I have been to pay my vifit to Mr. Her- 

 fchel, and according to my promife, proceed to give you 

 fome account of what I have feen ; and indeed he has fo 

 much originality about him, as well as natural ingenuity, 

 accompanied with great readinefs and dexterity, that to 

 enter into the detail would be far to exceed the bounds of a 

 letter ; I will therefore enter into the great outhnes, and fill 

 up as I can. You muft know that, till this vifit, I have 

 held the doftrine about telefcopes that I beheve is the com- 

 mon one ; that, having fixed upon a proportion that you by 

 experience find to do well in any one fpecies of telefcope, 

 what you are to expcft from any other fize of the fame 

 fpecies, is in proportion to the fquare root of the length ; 

 fo that increaCng the length four times, your telefcope will 

 allov; you to take an image of double the diameter ; every 

 point of it being illuminated with the fame quantity of light, 

 and painted with an equal degree of dillinftnefs and pre- 

 cifion. This idea and expeftation I carried with me to 

 Thornhill, and carried the fame to Clay-Hall ; but I did not 

 bring it back with me. Mr. Herfchel's doftrine will illuf- 

 trate his purfuits better than minute defcriptions. What- 

 ever his doftrine originally was, experience has taught him 

 that large furfaces of fpeculums are not to be ground and 

 poHfiied fo as to preferve fo accurate a figure as thofe of a 

 fmall or moderate fize ; he therefore divides the maximum 

 that telefcopes may be expefted to bring out, into three 

 diftinft clafTes ; firft, the greateft poffible degree of magni- 

 f)-ing power, where there is a fufiiciency of fight ; fecondly, 

 the greateft degree of diftinftnefs, where there is alfo a fuf- 

 ficiency of light, but where the natural fize of the objcft 

 does not require the greatefl. degree of magnifying power ; 

 thirdly, the greateft degree of light, where the objeifls are 

 naturally obfcure, whicn will afford difcoveries that cannot 

 Jse brought out cithpr by great degrees of magnifjing power, 



or a capacity of diftinftnefi, where, on tliefe aecountJ, ii 

 fufficiency of light is wanting. In conformity with this doci, 

 trine, his principal difcovenes have bivii made ujHiii the 

 (tars, where the greateft degree of niagnifyiiig powers have 

 been required, and ufed with his original telefcope of fivcn 

 feet focal length, which lie has pufliL-d to between fix and 

 feven thoiifai,d times. The givateti difcovenes liave alfo been 

 made with thefe, where the greateft diftinttnefs has been 

 required, and a moderate degree of magnifying power ; the v 

 diameter of the fpecuhim of this telefcope being no more 

 than 6A inches : and alfo, for tlie fame purpofe, he finds liii 

 ten-feet telefcopes applicable, the diameter being 9 inches'*' 

 but for obji fts naturally obfcure, lie can dift initly (ee an objeA 

 with his twenty-feet telefcope tlie diameter 19 inches, (which 

 is feldom charged with a magnifying power of more than 

 200 times,) which the others will not reach. With this te- 

 lelcojv; he is now and has been for fome time paft at work, 

 as he calls it, fweipmg the heaveas. The whole apparjiliii 

 can upon occafion be turned to any azimuth, but is chiefly 

 ufed with th« telefcope turning in the plane of the meri- 

 dian. The inferior or fpcculum end of the tube is fup- 

 ported immediately upon the ground ; the other end of the 

 tube is raifed and lowered by a tackle, fupportcd at top upon 

 a double equilateral triangle (or thereabouts) ; the obferver 

 is alfo hoifted up in a chair, that works on rollers, upon the 

 inclined legs of the triangle next the eye-glafs ; and the 

 eye-glafs is brought to anfwcr to this ftraigiit line by Aiding 

 the butt of the telefcope near the centre of the whole ma- 

 chine ; and by the fame means it can be put into a vertical 

 pofition. The raifing the chair and the flidiiig of the butt 

 are done by feparate tackles refpeftively, touched only f)c- 

 cafionally ; but the main tackle that raifts the telefcope, 

 when brought to itj intended elevation, that i^ polar dif- 

 tance, is worked by a diftinft motion, that caufes it to rife 

 and fall alternately through a fpacc of two degrees of the 

 meridian, which being done with fome degree of brilkncfs, 

 a plot in the heavens is examined at once of two degr"-^ 

 broad, the motion of the heavens in AR bringing on the 

 objedls in fucccftion. By way of regifter, large (heets of 

 paper are prepared, marked and numbered, being ruled hito 

 parallel long and crofs lines at a quarter of an inch diftance ; 

 a fmall fquare of this kind repreft-nting a quarter of a de- 

 gree in AR and declination: all thofe that are examined 

 being marked with a crofs, and thofe that have been feen, 

 but not fully examined, with a ftroke one way ; and when af- 

 terwards feen to fatisfaftion, the crofs is completed. The 

 place and fpecies of the objeft are alfo marked upon the 

 paper. In this operation, three perfons are concerned ; a 

 labourer works continually the handle backwards and for- 

 wards for performing the deftined range ; and in this he it 

 prevented from ranging too little or too much, by a fmall 

 piece of machinery, that ftrikes a bell at each end of the 

 range ; he alfo ftops on notice : and if any thing comes re- 

 quiring this notice, and the objeft to be pnrfued, the tele- 

 fcope can by an apparatus, which occafionally heaves it from 

 its meridian bearing, purfue it in right afcenfion for near a 

 quarter of an hour ; and that tliere may be no need for the 

 obferver's eye to be taken from the eye-glafs, an afiiftant 

 ( Mr. Herfchel's fifter) fits in an adjacent room with the 

 fquared fheet before her, who notes down and in a book 

 writes what is diSated. The time fhe has by the clock 

 facing her, and the polar diftance by a piece of machinery, 

 which continually (hews the degree and minute, and is 

 worked by a ftring aftuatcd by the telefcope in rifing and fall- 

 ing, which comes into the room, and winding round a barrel, 

 performs the requifite motions. The telefcope is fct to its 

 N n 2 altitude 



