TELESCOPE. 



cific gravity, wliicl. is afTumcd as bearing a due propor- 

 tion to the difperfivc power. Tluis, wlaii a piece of crown 

 and a piece of flint glafs are produced for an achromatic 

 obiea-glafs, the fpecific gravity is firft taken, and then 

 the tabulated numbers, corrcfponding to thcfe gravities, are 

 taken from the columns of the tables, and the work is put 

 in hand as foon as fuitable tools are felefted for producing 

 the curves : or rather, wlien tlie relative foci are determined, 

 the curves are fixed on in the tables that will fuit the aber- 

 rations in queftion, and that can alfo be produced by fuch 

 tools as are in ufe ; for the formation of a new grinding tool 

 is a ferious undertaking, that the optician will wiih to avoid. 

 But after all, the chierpra<ftical difficulty remains ; the fame 

 curves cannot always be worked to be exaftly fimilar, even 

 in the fame glafs, with the fame tools, and by the beft work- 

 men ; which circumftance leaves the nice calculator, in fome 

 rocafure, under the controul of his materials, and renders final 

 adjuftments indifpenfable. Thefe obfervations are corrobor- 

 ated both by the candid acknowledgment of TuUey, and by 

 the fubjoined extrafts, which we beg leave to tranfcribe from 

 the letters of our eftimable correfpondent Mr. G. DoUond. 



" The perfeftion of our objeft-glaflTes," fays Mr. DoUond, 

 *' is in a great degree promoted by the great pains we take 

 in feleding thofe glaffes that fuit each other the beft ; and 

 alfo in adjufting them very carefully : yet that is not every 

 thing that is necefTar^' to produce good objeft-glafles ; they 

 muft be correftly worked, and the glaffes be of perfeft and 

 proper quality. 



" With re{pe&. to the furfaces ufed in our various objeft- 

 glafTes, it would be almoft endlefs to enumerate them, as they 

 depend upon, and \ary with almoft every piece of glafs that 

 is ufed in their formation ; and there are fome nice points in 

 the method of working them, v.hich I (hould not wifti at 

 prefent to difclofe. Our ufual mode of proceeding is, in the 

 firft place, to calculate the proportions that are requifite for 

 the kinds of glafs that are to be ufed, and then to feleft from 

 our great number of tools thofe that come the neareft to the 

 furfaces determined upon ; and it frequently happens that 

 we have not any that will anfwer, particularly for the fphe- 

 rical aberration. We do not enter into thofe very nice cal- 

 culations that would be fatisfaftory to a theorift ; we only 

 aim at fomething near to what is required ; for to practical 

 men, it is always more eafy to produce what they wifh by 

 praftical methods. Mr. Short, the celebrated maker of 

 reflecting telefcopes, ufed to proceed by firft making his 

 large metal as nearly correft or parabolical as he could, 

 and then, from a number of fmall metals, to felect, by trial, 

 that which correAed the large one in the beft manner. 



" In all matters relating to the praftice of optics there is 

 much uncertainty, and it frequently happens that, with the 

 very beft endeavours, we cannot produce by the fame means 

 the fame effeft, where extreme correftnefs is required ; fo 

 that you may very readily conceive, that very exaft calcula- 

 tions, however requifite, will not always anfwer. In a rough 

 way of taking the focal lengths and furfaces of an achromatic 

 objeft -glafs, compofed of crown and flint glafs of the ufual 

 denfities, we fhould fay crown i : 3 and flint 2:3; the outer 

 furface of the crown fliorter than that which is next to the 

 flint, and the ftiorteft radius of the flint next to the crown ; 

 and the nearer it can be brought without touching in the 

 middle, the more perfeft will be the perform.ance ; though 

 this will in a great degree depend on the aberrating powers 

 of the glafles ufed ; for fometimes we find it neceffary to 

 make the crown nearly of equal radii. The French opti- 

 cians make ^he radii of the convex lens very unequal, and 

 place the ftiorteft radius next to the flint ; and inftead of 

 rrown they ufe Bohemian plate, which is nearly of the fame 



rcfrafting power, but of a different colour, their flint-glafs 

 being of a much lefs fpecific gravity than the Englifh. 



" The great barrier to further improvement, particularly in 

 the extenfion of the aperture, is the want of good glafs, which 

 circumftance has ever been lamented ; and from the excef- 

 fively increafcd duties, which aft againft the improvement 

 of every manufaAure, a prohibition is now likely to take 

 place altogether." 



In this hiftorical account of the invention and fucceflive 

 improvements of the telefcope, we have faid nothing about 

 the ingenious experiments of Dr. R. Blair, profefTor of aftro- 

 nomy in the univerfity of Edinburgh, which were made, 

 with a view to afcertain the difperfive powers of different 

 liquids, about the year 1787, and for this reafon, that we 

 confider any telefcope of which a hquid forms a conftituent 

 part, to be a temporary rather than a permanent inftrument. 

 Neither have we given Dr. Herfchel's labours fo prominent a 

 place in our narrative as they deferve, becaufe we fhall have 

 occafion to defcribe his reflefting telefcope, with reference 

 to its appropriate plate, in a fubfequent feftion of our 

 article. 



Befides the preceding improvers of the telefcope, feveral 

 pcrfons, chiefly amateurs, have taken out patents, either for 

 alterations in the appendages of this inftrument, or for pe- 

 culiar modes of ufing them for particular purpofes, with a 

 fliort notice of which we fiiall conclude this feftion of our 

 article. On the 4th of April, 1 79 1, Mr. Robert Blair, a 

 furgcon in the navy, took out a patent for fecuring to him- 

 felf the advantages to be derived from ufing a fluid medium, 

 in conjuiiftion with glafs, to correft the prifmatic aberration 

 in an objeft-glafs of a refra&ing telefcope, agreeably to the 

 experiments previoufly made on this fubjeft by Dr. Robert 

 Blair, as we have juft ftated. On the 26th of January, in the 

 year 1799, Mr. Cater Rand, of Lewes in Suftex, took out a 

 patent for " an improved military and naval telefcope, for 

 afcertaining diftanccs, and the fize and extenfion of objefts, at 

 fight, by means of a new micrometrical adjuftment." This 

 micrometrical telefcope, however, was nothing more than the 

 parallel wire micrometer, applied to a common pocket achro- 

 matic telefcope, in which a vernier fcale projeftcd from the 

 eye -piece, and indicated the quantity of the meafured angle 

 to the profeffed accuracy of 6"; but how the inftrument 

 was kept fteady enough without a ftand for the ufe of fuch 

 a micrometer, is not explained. Mr. Dudley Adams, of 

 Fleet -ftreet, optician, took out a patent, on May 30, iScwD, 

 for rendering telefcopes more portable ; the objeft of which 

 was to fecure the advantage to be derived from ufing tubes, 

 with flits made in fuch a way as to make them move fmooth- 

 ly, and yet without fliake, within one another. Mr. G. H. 

 Brown, fecretary to the Weftminfter fire-office, in Bedford- 

 ftreet, Covent-Garden, has defcribed, in the I ith volume of 

 the Repertory of Arts and Manufattures, a reflefting tele- 

 fcope, that always lies in a horizontal pofition ; and, receiv- 

 ing the rays of hght on an inclined plain mirror, having a 

 central perforation, and placed near the infertion of the eye- 

 tube, reflefts them to the large concave fpeculum, which, by 

 a fecond reflection, forms the image in the eye-tube. Benja- 

 min Martin conftrufted a reflefting telefcope in this way, 

 which he ufed in a vertical pofition for terreftrial objefts ; 

 and the only difference in the two conftruftions feems to be, 

 that in Martin's, the main tube was reclined when viewing 

 elevated objefts, fuch as the heavenly bodies, whereas 

 Brown's plain mirror has a vertical motion independently 

 of the main tube. They have neither of them come into 

 common ufe. 



Mr. Manton, gun-fmith, of Davis-ftreet, Bcrklcy-fquare, 

 London, took out a paterA on the 23d of January, 1810, 



for 



