SPECTACLES. 



the eye is defeftivc, snd by its fize or other conformstion, 

 parallel rays form their foci before they arrive at the retina, 

 then the perfon can fee very near objefts only. Such perfons 

 are faid to be near-fighted, or they are called myopes. 

 When the eye is flatter than ordinary, then the foci of 

 rays from pretty near objeft? are formed beyond the retina. 

 Perfons with fuch eyes are czAeA preJlyU ; they can adjuit 

 their eyes for objefts beyond a certain diitance only. The 

 latter is generally the cafe with old perfons ; but the eyes 

 of old perfons fometinies are incapable of adjuftment both 

 for very near and for very diftant objefts. This comes from 

 a rigidity or want of pliabihty in the parts. Thofe defefts 

 are frequently brought on or increafed by habit, as by the 

 conftant cuilom of viewing objefts either from too near 

 or from too great a diftance ; as alio by the ufe of improper 

 glalTes. 



Thofe imperfeftions may in a great meafure be remedied 

 by the ufe of proper glades or fpeftacles ; for fince, in near- 

 fighted perfons, the rays of light converge to a focus too 

 foon, 11/2. before they come to the retina, concave lenfes, 

 which diminifli the convergency, mult remove the imper- 

 fcftion. And for thofe who can fee diftant objefts only 

 with tolerable diltinftnefs, to'k. in whofe eyes the rays do 

 not converge foon enough, convex lenfes, which increafe 

 the convergency, muil remove the imperfeftion. 



When the defeft comes from rigidity, as in fome old 

 perfons, then thofe perfons require concave glafles for view- 

 ing diltant objefts, and convex glades for viewing near 

 objefts ; for their eyes want both adjuftments. 



The capability of adjuftment is greater or lefs in different 

 eyes, and it is frequently different in the two eyes of tlie 

 very fame perfon ; but in all eyes there is a limit, within 

 which vifion is not diftinft. This is called the limit of diJlinEl 

 •oifton ; and with fome perfons it is as diort as one inch, whilft 

 in others it exceeds twenty inches ; but in common it will 

 be found to lie between fix and ten inches. 



In Spain, and at Venice efpecially, fpcftacles have been 

 ufed with a different view from either of thofe above dated ; 

 all the people of note and fafliion there have them con- 

 tinually on their nofes : a fully, that has its fource in the 

 natural pride of thofe people, who value themfelves on a 

 profound wifdom ; and affeft to ftare very near at every 

 thing ; as if their eyes were weakened, and worn out with 

 excefs of attention. Vign. de Mary. 



F. Cherubin, a Capuchin, dcfcribes a kind of fpeftacle 

 telefcopes, for the viewing of remote objefts with both 

 eyes; hence called binocuU ; though F. Rheita had men- 

 tioned the fame before him, in his " Oculus Enoch et Elis." 

 See BiNocLE. 



- The fame author invented a kind of fpeftacles, with three 

 cr four glades, which performed extraordinarily. 



Speftacles were certainly unknown to the ancients. 

 Francifco Redi, in a very learned treatifc on fpeftacles, will 

 have them to have been invented in the 13th century, be- 

 tween the years 1280 and 131 1, (probably about the year 

 1299 or 1300,) and adds, that Alexander de Spina, a monk 

 of the order of Predicants of St. Catharine, at Pifa, firft 

 communicated the fecret, which was of his own invention ; 

 upon learning that another perfon had it as well as himlelf. 

 This hiftory is written in the chronicles of that convent. 



The fame author tells us, that in an old manufcript ftill 

 preferved in his library, compofed in 1 299, fpeftacles are 

 mentioned as a thing invented about that time : and that a 

 famous Jacobin, one Jourdon de Rivalto, in a treatife com- 

 pofed in 1305, fays exprefsly, that it was not yet twenty 

 years fince the invention of fpeftacles. He likewife quotes 



Bernard Gordon in his " Lilium Medicins," written the 

 fame year, where he fpeaks of a coUyrium, good to enable 

 an old man -o read without fpeftacles. 



Mufchenbroeck, Introd. vol. ii. p. 786, obferves, that it 

 is infcribed on the tomb of Salvinus Armatus, a nobleman 

 of Florence, who died in 1317, that he was the inventor of 

 fpeftacles. 



Du-Cange, however, carries the invention of fpeftacles 

 farther back ; alluring us, that there is a Greek poem in 

 manufcript in the late French king's library, which (hews, 

 that fpeftacles were in ufe in the year 1 1 Jo ; neverthelefs, 

 the Diftionary of the Academy dellaCrufca, under the word 

 occhiale, inclines to Redi's fide ; and quotes a padage from 

 Jourdon's Sermons, which fays that fpeftacles had not been 

 twenty years in ufe : and Salvati has obferved, that tbofc 

 fermons were compofed between the years 1330 and 1336. 



It is probable that the firft hint of the conllruftion and 

 ufe of fpeftacles was derived from the writings either of 

 Alhazen, who lived in the 12th century, or of our own 

 countryman Roger Bacon, who was born in 1214, and died 

 in 1292 or 1294. The following remarkable paffage occurs 

 in Bacon's " Opus Majus," by Jebb, p. 352. " Si vero 

 homo afpiciat literas et alias res minutas per medium cryf- 

 talli vel vitri, vel alterius perfpicui fuppofiti litcris, et fit 

 portio minor fpherx, cujus convexitas fit verfus oculum 

 et oculus fit in aere, longe melius videbit literas, et appare- 

 bunt ei majores. — Et ideo hoc inftrumentum ell utile fenibus 

 et habentibus oculos debiles : nam literam quantumcunque 

 parvam podunt videre in fufficienti magnitudine." Henee, 

 and from other padages in his writings much to the fame 

 purpofe, Molyneux, Plott, and others, have attributed to 

 him the invention of reading-glalies. 



Dr. Smith, indeed, obferving that there are fome miltakes 

 in his reafoning on this fubjeft, has difputed his claim. See 

 Molyneux's Dioptrics, p. 256. Smith's Optics, Rem. 

 86—89. 



The elfential and extenfive ufe of fpeftacles, which 

 affords comfort to fo great a number of individuals, who 

 would otherwife be a burden to themfelves and to fociety, 

 is an inftance of the great ufefulnefs of the fcience of 

 optics. 



No pains have been fpared to render fpeftacles as perfeft 

 as poffible, and a variety of contrivances have been from 

 time to time offered to the public. Speftacles have been 

 made with two lenfes for each eye ; alfo the lenfes have 

 been made plano-convex, or plano-concave, or of other 

 fhapes ; but, upon the whole, fingle lenfes, either double 

 concave, or double convex, of clear glafs, well polilhed and 

 regularly formed, are the bell. 



When the eyes of perfons firft begin to be affefted by 

 age, the opticians furnidi them with fpeftacle lenfes, of 

 about forty inches focus, which glades are therefore called 

 number i!t, or glades of the firlt fight; inz. for the fight 

 when it firft begins to be impaired by age. But there is 

 confiderable difference between the focal diftances of fpec- 

 tacles. N° I, made by different opticians. When the 

 focal length is about fixteen inches, the lenfes are called 

 N' 2. About twelve inches are the focal length of N° 3. 

 Ten inches are what they call N° 4. Nine inches are thofe of 

 N" 5. Eight inches give the focal length of N° 6. Seven 

 inches are the focal length of N° 7. Six inches are the focal 

 length of N° 8. And fometimes they make fpeftacles of a 

 focus Ihorter ftill. Concave fpeftacLs are alfo named by 

 fimilar numbers. 



In choofing fpeftacles, aftual trial is the belt guide; 

 but care muft be had to ufe fpeftacles that do not magnify 



more 



