TELESCOPE. 



defideratum in aihonomical iiillriinn.Mits to obtain an unobj(?c- 

 tionable, and at the fame time an cal y metliod of mcafuring, 

 by a micromctrical tclefcope, the diftance between two very 

 fmall ftars, fo near to each other as to be called double ilars ; for 

 when fo much extraneous light is admitted into the tclefcope 

 as to fliew the fpider's lines, or fcalcs of a mechanical micro- 

 meter, the minute ftars vanifli ; and, when optical micro- 

 meters with double images are ufed, the light is fo divided 

 between the images, that tlic ftar alfo vunillies, in this cafe, 

 from want of light. Dr. Malkelvne's prifmatic micrometer . 

 is, perhaps, lefs liable to this objeftion than any other, but 

 is not yet brought into common ufe. 



Before we difmifs this part of our article, we beg leave 

 to ftatc, that about a hundred years ago, De la Hire con- 

 trived a method of giving different values to a pair of wires 

 fixed in the focus of the principal object -glafsot a tclefcope, 

 by means of another moveable objed-glafs ; and alfo that 

 about the year 177 I, Mr. James Watt, celebrated for hi» im- 

 provements on the fteam-engine, not only contrived the fame 

 thing, but aftually made the meainremcnt by a longitudinal 

 fcale marked on his tube, nearly as done by Dr. Brewiler. 

 Mr. Watt's claim to originalily, we believe, is undifputcd, 

 and may be proved both by his letter on the fubjeC\ to Mr. 

 Smeaton, written near that time, which letter is lull in exiR- 

 ence ; and alfo from the circumftance of his having about 

 the fame time defcribed his nevi- inftrument to the late Mr. 

 Ramfden. With thefe prior inventions, however, we are well 

 affured Dr. Brewfter was not acquainted at the time of his 

 taking out a patent, in conjunftion with Mr. Harris, the prc- 

 fent vender of the patent telefcope ; and therefore he alfo is 

 entitled to the merit of originality ; and moreover appears to 

 have the fole right to the idea of converting it into a gene- 

 ral micrometer, of applying it to the divided objetf-glafs, 

 and of converting a Gregorian or Caflegrainian teleicope 

 into a micrometer, without any additional lens or mirror. 

 Mr. Watt never made much ufe of this micrometrical tele- 

 fcope, the impreflion on his mind being, that the fcale ought 

 not rigidly to be a fcale of equal parts, which Dr. Brewiler 

 has fince demonftrated to be the cafe, and his line of bufinefs 

 not leading him to finifh all the adjuftments for real ufe. 



This ingenious gentleman had previoufly, ik'z. in the year 

 l770,conftru(3;ed a micrometer, with a pair of parallel horizon- 

 tal wires, crolTedby a fingle wire at right angles, in the prin- 

 cipal focus of an ordinary telefcope, which afted as a micro- 

 meter for determining diftances at one ftation thus ; a twelve- 

 feet rod had a circular difc of wood, eight inches in dia- 

 meter, painted white, that was crolTed by a red horizontal 

 line of an inch in width, which difc was made to Hide along 

 the rod, while another fimilar difc was fixed fall about a 

 foot above the ground, when the rod ftood in a vertical 

 pofition ; then, at any unknown diftancc, the fllding difc 

 was lowered till one wire of the teleicope covered its red 

 line, while the other wire covered the red line of the fixed 

 difc ; and then a fcale upon the rod, graduated by experi- 

 mental meafures, indicated the dijlance by infpeftion to 

 within fi, part of the whole. This inftrument was ufed 

 with a telefcope of only twelve inches focal length, and an 

 eye-glafs of an inch and a half focus, fo that the magnify- 

 ing power was only eight times with this inftrument. 

 The furvey of the intended canals of Crinan, Gilp, and 

 Tarbert, was made by Mr. Watt in 1772, as well as the 

 furvey of the canal running from Invernefs to Fort William, 

 called the Caledonian canal, in 1773. This contrivance 

 was fhewn to feveral friends, and amongft them to Mr. 

 Smeaton, though an account of it has never before been 

 publiftied. In the year 1778, a Mr. CJreen applied to the 

 Society of Arts in the Adelphi for a premium for a fimilar 



invention ; on which occafion, Mr. Smeaton advifed Mr. Watt 

 to attend at the Society's rooms, to Itate the priority of hit 

 claim, which was accordingly done ; but as Green's telefcope 

 had more magnifying power than Mr. Watt's, riz. 40, and 

 was fuppofed on that account to be more accurate iii de- 

 termining diftances, the claim of priority was ceded in Green's 

 favour, and Mr. W.att's invention was fuffered to go wn- 

 noticed by the Society. 



A double-image micrometer was alfo invented by Mr. 

 Watt in the year 1 77 i , which, as it has never been defcribed, 

 we fhall make no apology for introducing here, in corapany 

 witli his other ingenious inventions. This inftrumer.t con- 

 fifted of a circular difc of glafs, whofe plane fides were not 

 ftrictly parallel, but formed with each other an angle of one 

 or two degrees, fay a wedge or prifm of one /\ s 

 or two degrees. "This difc or prifm w;ls cut ( _ \ 

 by a diamond, at right angles to the Hope of \^ ] 



the prifm, into two unequal fegments thus; \^ / 



The lefl'er piece. A, was fixed, while the Larger piece, B, 

 was moveable upon the diamond-cut line, as . j. 

 upon an axis or hinge, as leen in this plan, in IWi--, — =3 

 which the dotted lines Ihew one of the pofi- X;. 



tions into which B may be moved. When the 

 two fegments remain in the fame plane, they refraft all the 

 rays,«^)ichpafsthrough them equally ; but A remaining lixed, 

 and B moving upon a centre, as drawn in dotted lines, the rays 

 which pais through B, will be more refrafted than thofe 

 which pafs through A, and thus will vary with the angle 

 which B makes with A. This divided prifm being fixed in 

 the focus of (or before) the objcft-glafs of a telefcope, two 

 images are formed of every objeft by which its diameter may 

 be meafured. An ind<'X and divided feftor of a circle fervc to 

 meafure the comparative refraftions. This inftrument, how- 

 ever, has the fault, that the divifions arc not equal pans for 

 equal angles, and moreover the prifms would require to be 

 achromatic, where high magnifying powers are required. 



It was not till the year 1777, that the abbe Bofcovjch 

 publilhed ail account ol the prifmatic micrometer of the abbe 

 Rochon made of rock-crylfal, with double refraction, and 

 alio of his own improvement, or fubftitulion of glafs prifms, 

 where one of them revolved round ail axis of motion like 

 Mr. Watt's; nor was it tiil the fame year that Dr. Maf- 

 kelyiie publiftied his account of a prifmatic microir.etcr, that 

 meafures a fmall angle by the refracting angle and the dif- 

 tance of the prilnis from the focus of the objeft-glafs con- 

 jointly ; where the length of the telefcope was the fcale of 

 meafurement. See Phil. Tranf. 1777. 



Refletling. — We proceed in the next place to defcribe the 

 conftrudlion of reflefting telefcopes. Fig. 1. Plate XXX. 

 (hews the figure of a refletling telefcope of either the 

 Gregorian or Caffegrainian kind, for their external appear- 

 ance and mode of ufing are the fame, though we have 

 fhewn that their fmall fpecula are differently formed. After 

 the minute defcription that we have given of the ftands for 

 achromatic reflefting telefcopes, we may avoid prohxity, by 

 giving a lefs minute account of t'lofe that have been appro- 

 priated to reflcftors, where the ufes of the fame parts are 

 the fame : A B is the main tube of a refletling teleicope of 

 moderate fize, which may be either with or without a finder, 

 as the power may require, mounted on the tripod G F ; at 

 A is the open aperture, and a little fliort of it, within, is the 

 fmall fpeculum, drawn in or out by the fcrew C, which is 

 connedlcd with a longitudinal bar of metal, into which the 

 heel-piece of the ftem of the fmall fpeculum Aides, fo as to 

 be taken away or put in at pleafure ; within the interior end 

 B of the large tube is the perforated large fpeculum, 

 always concave, and of a proper figure to fuit the face of 



the 



