TELESCOPE. 



prifmalic aberrations was not abawloned. We tiave already 

 alTertcd, under our article Circle, that Cheftcr More 

 Hall, efq. of More Hall in EfTex, fo long ago as in the 

 year 1729, conilrudcd telefcopes of different glaffes ; fome 

 of which have been prefervcd, and found, on examination, to 

 be of the achromatic kind, though not known by this de- 

 fignation. But as we are not in pofTcffion of any record 

 refpefting the invention and mode of conltrufting fucli 

 telcfcope, we do not prefume to fay that this was the arche- 

 type of the modern achromatic inftrument ; and, therefore, 

 \vc do not conlider it as detradting from the merit of the 

 philofophic optician, who afterwards deduced the principles 

 of the invention from accurate and ingenious experiments, 

 and made known his fuccefsful application of them at a time 

 when his claim to originality might have been difputed, if 

 the prior invention had been then divulged. 



It was not till about the year 1747 that Euler, not 

 knowing what had been done by Hall, and profiting by a 

 hint that had been fuggefted by fir Ifaac Newton, conceived 

 the plan of conftrutting an objeft-glafs of two iuch ma- 

 terials, of different refractive powers, as might counteract, 

 by repeated refraSions, the- diiperfioii of the differently re- 

 frangible rays, and thus bring all the rays into one focal 

 point, fo as to admit of a highly magnifying eye-piece. 

 Accordingly two objeft-glaffes were fo incloied in a box, as 

 to include clear water between them, to be ufed inilead of a 

 fingle lens ; and though the experiment failed of fuccefe, 

 the memoir, written by Euler on the occafion, attracted 

 the attention of the late Mr. J. DoUond, mathematical- 

 inftrument maker of London, who foon after fet about 

 making experiments, as Newton had done, to afcertain if the 

 refraflive and difperfive powers of various tranfparent fnlj- 

 ftances are in a conftar.t ratio, with the view of compound- 

 ing, which he at length effefted, an objed-glafs that would 

 bring the rays of light tranfmittcd through it to a focal 

 point, without the prifmatic aberration. 



In the memoir which Euler had written, and which was 

 publjfhed in the BerHn Memoirs of 1747, he affumed that 

 the indices of refraftion might be expreiTed by the powers of 

 a certain invai-iable root, and that the exponents of thofe 

 powers are proportional for the different rays of lisrht pafling 

 through different media. This paper coming to the hands 

 of the ingenious DoUond, excited his attention ; and in the 

 year J753 he addreffed a letter to James Short, " concerning 

 a miftake in M. Euler's theorem for correfting the aber- 

 rations in the objeft-glafTes of refrafting telefcopes ;" which 

 letter was pubhfhed in the Philofophical Tranfaftions of the 

 fame year ; the objeA of which was to prove that Euler had 

 affumed an hypothefis, as the bafis of his calcuLations, which 

 was contrary to both realon and experiment, or, as Short 

 obferves in his accompanying letter, " contrary to the efta- 

 bhfhed principles of optics." To this Euler replied, and 

 maintained that his optical principle was a true law of na- 

 ture ; but the practical tcfl of its truth was wanting, the uje 

 that it was intended to be put to. 



In 1754, the Swedifh geometer KHngenfliema took up 

 the fubjeft, which now attrafted the attention of various 

 mathematicians, and attempted to prove that the Newtonian 

 principle, oppofed to Euler's, is in fome extreme cafes irre- 

 concileable with natural phenomena, and tlierefore ought not 

 to be received as a law of nature. DoUond, therefore, thus 

 oppofed, had recourfe to aftual experiment, agreeably to 

 Newton's plan of philofophizing ; and, rejefting the pro- 

 pofal of putting water between two menifci, with a view of 

 <;orredling the prifmatic aberrations by a number of refrac- 

 tions, proceeded to inflitute a regular feries of experiments, 

 ■n order to determine what could be done by the oppofite 



refratlions of different diaphanous media; and as tliefe ex-* 

 periments were the foundation of all the improvements that 

 have followed in the conllruCtion of colourlefs, or what Dr. 

 Bevis denominated achromatic, objedl-glafTes of a refracting 

 telefcope, we fhall here introduce a fummary account of 

 them. 



In the iirft place, DoUond contrived to form a hollow in- 

 verted pyramid with two oppofite fides of glafs, asinjff. I, 

 Plate XXIV. yljlrotwtmcal Inftruments, and placed in an in- 

 verted pofition, within a triangular and equilateral prifm of 

 glafs, to reft as reprefented in the figure. The veffel was 

 then filled with clear water, and a ray of light made to pafs 

 through both the water and glafs prifm : the angle at the 

 junction of the glafs plates, clofing the veffel, was capable 

 of enlargement or diminution ; and the glafs fides were made 

 to recede or approach, until an objetl feen through the water 

 and glafs prifm was in its true place, i. e. until the refrac- 

 tion of the water balanced the oppofite refraction of the 

 glafs. The refult of this experiment proved contrary to 

 what had been expetted from Newton's experiments, I'/'z. 

 an external objeft feen through this compound prifm was 

 fringed with colours. But to be quite fure that there was 

 no deception in the appearance, a glafs prifm, formed to an 

 acute angle of only 9 degrees, was fubftituted, which was 

 alfo more convenient for the experiment, and then the 

 veffel was clofed, as in Jig. 2, until the oppofite refradlions 

 balaiiced each other as before ; but ftill the objeft viewed 

 through the compound prifm was tinged with the prifmatic 

 colours. The mean rays in thefe trials proceeded in a 

 flraight hne, after quitting the iecond wedge of water ; but 

 the extreme rays were deflefted, or turned refpeftively out 

 from exaft parallelifm. 



After having thus afcertained that equal and oppofite re- 

 fractions of glafs and water will not dellroy the colours, the 

 author varied the experiment, by altering the wedges of 

 water, till he found that the refraction occafioned by the 

 water muft be to that occafioned by the glafs as 5 : 4, be- 

 fore the colours will difappear. The next ftep was to purfue 

 the proportions thus alcertained, in the conflruftion of an 

 objedl-glafs containing water ; but after uniting a deep and 

 double convex lens of pure water with a concave one of 

 glafs, the objeft feen through the tclefcope with this com- 

 pound objeft-glafs was indeed free from colour, but by no 

 means fo diftinft as was defired, and confequently the 

 fpherical aberration yet remained. This tclefcope was made 

 in 1757, and ferved to prove that the feparation of the ex- 

 treme rays, or what has fince been called the difperfive 

 poiver, in the cafe of an union of glafs with water, is not 

 proportioned \.o the viean refradion ; as fir Ifaac Newton had 

 afferted it to be, in the fame experiment (fee Newton's 

 Optics, p. 112, 3d edit.): confequently the idea muft now 

 have occurred of trying other diaphanous fubftances with 

 different refradtive powers, to fee what the difperfion would 

 be in them. After an interval of fome time, during which 

 different kinds of glafs were procured, the ingenious and 

 perfevering artift found, for the firft time in the year 1757, 

 that the difperfive power of the cryftal or white JUnt-^^Ss 

 was greater than that of the Enghlh rrown-glafs, and alfo 

 that the power of the latter was very fimilar to that of the 

 Venice ftraw-coloured glafs. He determined, therefore, to 

 try a wedge of flint-glafs, and another of crown-glafs, 

 formed to different angles, as '^^ Jig- 3, until, when reverfed, 

 their oppofite refraftions were equal ; which equality took 

 place, when their angles were refpectively 25 degrees and 

 29 degrees ; in which cafe, the fines of half the angles, or 

 the indices of their refraftions, were 216 : 250, or nearly as 

 19 ; 22. But though the direftion of the pencil of light 



