SPECTACLES. 



quently overcome. But when perfons in the higher ranks of 

 life difcover that their difcernment of diftant objefts is lefs 

 quick or lefs correft than that of others, however fliglit the 

 difference may be, fuch perfons, influenced perhaps by 

 fa/hion more than by neceflity, recur immediately to tlie ufe 

 of a concave glafs ; and thus their eyes become fo fixed in 

 the Itate requiring its afliftance, that the recovery of diltant 

 vifion is rendered afterwards extremely difficult, if not quite 

 impuflible. The number of perfons who aftually need this 

 adiflance, is comparatively very inconfiderable ; and there- 

 fore it fiiould more generally be avoided. The misfortune 

 refulting from the ufe of concave glaffes is this, that the 

 near-fightednefs is not only fixed by it, but from a liabit of 

 inquiry with regard to the extreme perfeftion of vifion, 

 frequent changes are made for glaffes that are more and more 

 concave, until at length the near-fightednefs becomes fo 

 confiderable, as to be rendered ferioufly inconvenient and 

 afflifting. It ought to be remembered, that, for common 

 purpoles, every near-figiited eye can fee with nearly equal 

 accuracy through two glaffes, one of which is one number 

 deeper than the other ; and though the fight be in a flight 

 degree more aflilted by the deepeft of thefe than by the 

 othei', yet on its being firft ufed, the deepeft number always 

 ccafions an uneafy fenfation, as if the eye were ftraincd. 

 r, therefore, the glafs that is moft concave be at firft em- 

 noyed, the eye, in a httle time, will be accommodated to 

 '., and then a glafs one number deeper may be ufed with 

 iiuilar advantage to the fight; and if the wifh for enjoy- 

 ing the moft perfeft vifion be indulged, this glafs may foon 

 be changed for one that is a number ftill deeper, and fo ni 

 fucceflion, until at length it will be difficult to obtain a glafs 

 fufficiently concave to afford the affiftance which the eye 

 requires. Mr. Ware obferves, that molt of the near-fighted 

 perfons whom he has known, have had the right eye more 

 near-fighted than the left ; and he thinks it rot impro- 

 bable, that this difference between the two eyes has been 

 occafioned by the habit of ufing a fingle concave hand-glafs ; 

 which, being commonly applied to the right eye, contributes 

 to render this eye more near-fighted than the other. In 

 fome, cafes children have manifelted near-fightednefs almoft 

 as foon as they begin to take notice of furrounduig objcdls ; 

 but this, which is occafioned by feme degree of opacity in 

 the tranfparent parts of the eye, is very different from that 

 ftate of the eye to which the term myopic, or near-fighted- 

 nefs, is ufually applied ; and by which is fimply meant too 

 great a convexity either in the cornea or in the cryftaUine, 

 in proportion to the dillance of theie parts from the retina. 

 So far from difcouraging the ufe of fpeAacles in fuch cales, 

 it is neceilary to recur to them, as without them eiiildren 

 could not profecute their learnmg with eafe or convenience. 

 Extreme near fij^htednefs is fometimes occafioned by an evi- 

 dent change in the fpherical figure of the cornea, and its af- 

 fumption of a conical fhape. This morbid ftate of the 

 cornea admits of no amendment by any glafs. The cornea, 

 in moft cafes of this kind, is preternaturally thin ; and it is 

 not unfrequently accompanied with fymptoms of general 

 debility, under which laft circumftance, chalybeate medicines, 

 and bracing applications to the eye, have afforded confider- 

 able benefit. Near-fightednefs, our author fays, is fcldom 

 alike in the two eyes, and he has obfervcd a few cales, in 

 which one eye of the fame perfon has had a near, and the 

 other a diftant fight. Dr. Porterfield has fuggelttd, that 

 the pupils of near-fighted perfons are more dilated than 

 thofe of others ; but this does not accord with Mr. Ware's 

 obfervations in fuch cafes. This ingenious writer, whofe 

 remarks on all circumllances relating to tlie eye merit pecu- 

 liar attention, obferves, that near-fight«dnel8 has no de- 



pendence on the greater or Iraaller degree of convexity pof- 

 feffed by the cornea, when this circumftance is confidered 

 alone ; fince the length of the axis of the eye from the cor- 

 nea to the retina, and the greater or fmaller degree of con- 

 vexity in the cryftalline humour, muft be alio regarded 

 before the diftance of accurate vifion can be determined. It 

 is alfo no lefs evident, that near-fightednefs is not neceffarily 

 occafioned by a morbid protruCon of the whole eye ; fince 

 fome perfons are born with eyes of this defcription, and 

 others acquire the peculiarity, when further advanced in hfe, 

 in confequence of a morbid accumulation of adeps at the 

 bottom of the orbit, without either of them being more 

 near-fighted than thofe who are free from this imperfeftion. 

 It is a faft often noticed, that old perfons, who have been 

 long accuftomed to ufe convex glaffes of confiderable power, 

 have recovered their former fight, even at the advanced age 

 of 80 or 90 years, and have then had no further need of 

 them. Tliis amendment is afcribed by Dr. Porterfield to a 

 decay of adeps at the bottom of the orbit ; but Mr. Ware 

 attributes this remarkable revolution in the fight of old per- 

 fons to an abforption of part of the vitreous humours ; in 

 confequence of which, the fides of the fclerotica are preffed 

 inward, and the axis of the eye, by this lateral preilure, is 

 proportionably lengthened. This kind of alteration is alfo 

 fufficieiit to explain the reafon, why fuch aged perfons re- 

 tain the power of diftinguifhing objects at a diftance, at the 

 fame time that they recover the faculty of feeing thofe that 

 are near ; fince the lengthened axis of the eye leaves the 

 power by which it is adjufted to fee at different diftances, 

 precifely in the fame ftate in which it was before the lengthen- 

 ing of the axis took place. The faculty, fays Dr. T. Young, 

 of feeing at different diftances, is produced by a power in the 

 cryftalline humours to become more or lefs convex, accord- 

 ing as the object is more or lefs diftant from the eye. 



Although old perfons lofe the power of diilinguifhing 

 corredtly near objefts, and require for this purpofc the aid 

 of convex glafies, they ufually retain the figlit of thafe that 

 are diitant as well as when they were young. Inftances, 

 however, occur of perfons advanced in life, who require the 

 aid of convex glaffes to enable them to fee near, as well as 

 diftant objefts. From the cafe of perfons who have had 

 the cryftalline humour removed, Mr. Ware infers, that this 

 humour is indifpenfably neceffary to enable the eye to fee at 

 different diftances. Such perfons alfo have lefs power to 

 afcertuin the diftance of an object when they look through 

 a convex glafs than when they view it without this affiftance ; 

 and accordingly fuch perfons feldom make ule of glaffes 

 when they are walking ; and the inconvenience of glaffes is 

 particularly experienced when tiiey defcend a fliglit of fteps, 

 or pafs over uneven ground. Our author obferves, that 

 near-fighted perfons do not appear to poffefs the fame extent 

 of vifion that is enjoyed by thofe vvhd have a diftant fight, 

 and he verifies the obfervation by a ftatemtnt of his own cafe, 

 whofe range of diftinft vifion did not extend further than 

 an inch and a quarter in either eye, and within thefe dillances 

 lie always held a book when he read. He recommends the 

 following rule for determining the concavity of a glafs that 

 is beft adapted for near-fighted perfons, to thofe wlio are 

 unable, from diftance or any otlier cauic, to fuit themfelves 

 at the fhop of an expert optician. The rule is this : multi- 

 ply the diftance at which the perfon reads with eafo, (which 

 in our author's cafe, with his left or beft eye, was five inches,) 

 by that at which he wiflics to read, which may be ftated at 

 12 inches ; divide the produdt, 60, by 7, the difterence be- 

 tween the two, and it leaves nearly 9 inches for the focus of 

 the concave glafs that Ihall produce the defired eftedt. Tliis 

 glafs anfwers to that fold under the name of N" 6 ; and this 

 3P 2 is 



