SPECTACLES. 



and if all the other defefts and infirmities of old age could 

 be as eafily and cheaply remedied, it would be worth while 

 to live a good deal longer." 



Mr. John Richardfon, an optician of London, obtained 

 a patent in 1797 for an improvement in fpeftacles, which 

 confiils in applying extra glafles before the common fingle 

 lenfes, and fitted with joints, fo that the extra glafles can 

 at pleafure be turned up out of the way, either on one fide, 

 or upwards. Tiie advantage of this plan is Hated to be, 

 that It precludes the neceffity of two pairs of fpeftacles for 

 different views, but that any two different powers of fight 

 may be inftantly obtained : for inftance, the fpeftaclcs may 

 be fitted with fuch lenfes as are moft proper to enable the 

 wearer to fee a profpeft in walking or riding ; but by turn- 

 ing down the extra lenfes, the fpeftacles will magnify in a 

 greater degree, and be proper for reading or writing ; and 

 by the fame means, coloured glaffes may be applied, to be 

 occafionally turned down over the lenfes of the fpeftacles, 

 to walk in the luii-diine. 



Mr. John Ifaac Hawkins fome years ago introduced a 

 new kind of fpeftacles, provided with extra arms or joints, 

 which pafs upwards over the temples, in addition to the 

 ufual jointed arms which hold the fpeftacles, by palhng 

 horizontally round the head. By thefe additional arms the 

 fpeftacles are borne altogether from the head, without rell- 

 ing upon the nofe, which occafions an unpleafant fenfaticn 

 to thofe who firft begin to ufe fpeftacles, although it 

 wears off in a few days ; but the advantage of Mr. Haw- 

 kins's plan is, that the glaffes, without occafioning any 

 preffure on the temples, can be placed at any diflance from 

 the eye, and inclined in any direftion, as is found moft con- 

 venient for the view. 



Dr. Hyde Wollafton's perifcopic fpeftacles are now 

 much in ufe, and are fo called from the facility which they 

 afford the wearer of looking round at various objefts. This 

 invention, for which he obtained a patent in 1804, is in- 

 tended to obviate that defeft of the common fpeftacle-sflaffes, 

 which we have before mentioned, iiiz. that no objects ap- 

 pear diftinft through them, except fuch as are feen through 

 the centre of the glafles, or nearly fo, and that the vifion is 

 indiftinft, in proportion as the part of the glals through 

 which the rays pafs is farther from the centre": this is moft 

 particularly obfcrvable in glafles of fliort focal diftance, and 

 confiderable magnifying power. It is on thi^ accomit that 

 opticians have lately made and recommended fpeftacle-glafles 

 of lefs diameter than thofe formerly in ufe, thinking that 

 the extreme parts of the field of vifion, which from indif- 

 tinftnefs were of little ufe, might be fpared without much 

 inconvenience. But this alteration in the fize of the glaffes 

 can fcarcely be called an improvement, fince for one defeft 

 it only fubitituted another fcarcely lefs objeftionable. 



Dr. Wollafton, having confidercd that the portion of 

 any glafs employed in any one pofition of the eye is but 

 fmall, perceived, that by making the fubftance of a glafs 

 curved, in the manner of a hollow globe, each portion of it 

 might be fituated nearly at right angles to the direftion of 

 the fight, and would thereby render lateral objefts diftinft, 

 without impairing the diftinftnefs at the centre. 



To illuftrate this ; fuppofing the eye to be placed in the 

 centre of any hollow globe of glafs, it is plain, that ob- 

 jefts would then be feen perpendicularly through its furface 

 in every direftion : confequently, the more nearly any 

 fpeftacle-glafs can be made to furround the eye, in the man- 

 ner of a globular furface, the more nearly will every part of 

 It be at right angles to the line^of fight, the more uniform 

 will be the power of its different parts, and the more com- 



pletely will the indiftinftnefs of lateral objefts be avoided. 

 It is evident, that no other rays than thofe which pafs 

 through the centre of a lens can be at right angles to 

 both furfaces ; but it is equally clear, that when any fmall 

 oblique pencil of rays makes equal angles with the two fur- 

 faces of a thin lens, its inclination to each furface will be 

 fo fmall, that its focal length will not fenfibly differ from 

 that of a central pencil. Upon this principle Dr. Wollaf- 

 ton's fpeftacle.glaffes are made, of a form not heretofore 

 ufed for that purpofe ; the outer furface of each glafs, or 

 that which is fartheft from the eye, being fpherically con- 

 vex, and the inner furface of the fame glafs, or that which 

 is neareft the eye, fpherically concave. A glafs of this 

 fort, adapted for a fhort-fighted perfon, will have the interior 

 and concave furface of each glafs more curved than the ex- 

 terior and convex furface of tlie lame glafs, by which the 

 rays of light palling through the fame are diverged, and the 

 degree of curvature of the interior and concave furface is 

 to be increafed in proportion as the perfon who is to ufe 

 the glafles is more Ihort-fightcd. On the contrary, forlong- 

 fighted pcrfons, the form of each glafs mull be fuch as tf> 

 have the exterior and convex furface thereof more curved 

 than the interior and concave furface of the fame glafs, 

 by which means the rays of light pafTing through the famx- 

 are converged ; and the degree of curvature of the exterior 

 and convex furface of the glafs is to be increafed in. propor- 

 tion as the perfon to ufe the glafs is more long-fighted. 



With a view of obtaining the fame effefts as Dr. Wollaf- 

 ton's perifcopic fpeftacles, a new fpecie? of glaffes have been 

 lately introduced at Paris, made by Chamblant, optician. 

 Rue Baffe, Porte St. Denis ; they are CaWed con/erves : thefe 

 glaffe.s are ground to cylindrical furfaces, inftead of the 

 fphericai furfaces which have hitherto been employed for 

 fpeftacles and lenfes of all kinds : the oppofite furface? of 

 each glafs are fegments of two different cylinders, inclined 

 to each other at riglit angles ; that is, the axes of the two 

 cylinders are fituated in parallel planes, but in direftion 

 they lie at right angles to each other, and in confequence, 

 the direftion in which one furface of fuch a glafs is curved, 

 is at right angles to the direftion in which the other fide is 

 curved. On making trial of a pair of thefe glaffes, we 

 found them very perfeft in the field of view, prefenting a 

 very diilinft vifion of objefts viewed through the outer 

 parts of the glafs, as well as thofe viewed through its cen- 

 tral parts : and the magnifying power of the whole field as 

 nearly equal as the eye could judge. 



The inventor has obtained a patent in France for this in- 

 vention, and makes fpeftacles of all degrees of magnifying 

 power. He propofes to apply iimiiar lenfes to telefcopes, 

 microfcopes, and other optical inflruments, but we have not 

 had an opportunity of examining any of them. 



Speftacles are made in great varieties of forms, and orna- 

 mented according to fafhion, or to the fancy of the wearer. 

 The frames are made of all kinds of materials, fuch as gold, 

 filver, fleel, pearl, tortoife-fhell, ivory, bone, and wood. 

 The lenfes are ufually made of glafs, but the better forts 

 are made of natural cryftal, and are known by the name of 

 pebble-fpeftacles, but in reality moft of the fpeftacles fold 

 under that name are made of a hard fort of glafs. 



It is obferved by Mr. Ware (Phil. Tranf. for 1813, I 

 part I.) that (hort-fightednefs ufually comes on between the ■ 

 ages of ten and eighteen. This defeft of fight, as foon as 

 it is accidentally difcovered, is little regarded by perfons in 

 the inferior ftations of fociety, and no means are ufed for 

 correfting it; hence by increafed exertions that are made for 

 diftinguifhing diftant objefts, the imperfeftion is not unfre- 

 II q'lently 



