SQUINTING. 



ftand over his nofe, witli a half circle of the fame metal to 

 go round his temples ; thefe were covered with black filk, 

 and by means of a buckle behind his head, and a crofs-piece 

 over the crown of his head, this gnomon was managed fo as 

 to be worn without any inconvenience, and projefted before 

 his nofe about two inches and a half. By the ufe of this 

 gnomon he fi)on found it \eh inconvenient to view all objefts 

 with the eye next to them, initead of the eye oppolite to 

 them. 



" After this habit was weakened by a week's ufe of the 

 gnomon, two bits of wood, about the fize of a goofe-quill, 

 were blackened all but a quarter of an inch at their fum- 

 mits : thefe were prefented for him to look at, one being 

 held on one fide the extremity of his black gnomon, and the 

 other on the other fide of it. As he viewed thefe, they 

 were gradually brought forwards beyond the gnomon, and 

 then one was concealed behind the other. By thefe means, 

 in another week, he ci'uld bend both his eyes on the fame 

 objeft for half a minute together. 



" By the praftice of this exercife before a glafs, almoft 

 every hour in the day, he became in another week able to 

 read for a minute together with his eyes both direfted on 

 the fame objefts ; and I have no doubt, if he has patience 

 enough to perfevere in thefe efforts, but he will in the courfe 

 of fome months overcome this unfightly habit." Phil. 

 Tranf. vol. Ixviii. p. 89. 



Philofophers, as well as phyficians, have adopted various 

 hypothefes, in order to account for this defeft in vifion. It 

 has been generally fuppofed to proceed, according to the 

 preceding ttatemeat, from the want of a due correfpondence 

 in themufclesof theeyes, which not aftingin a proper concert 

 with one another, are not able to point both eyes at one 

 objeft. M. de la Hire apprehended, that this diforder pro- 

 ceeded from a defedl in the eye itfelf having the moft fenfible 

 part of the retina not placed in the axis of the eye, but ae 

 fome diltance from it on one fide or the other ; and that, 

 therefore, not the axis, but this more fenfible part of the 

 retina is turned towards the objeft on which the axis of the 

 other eye is fixed, fo that both axes are not direfted to the 

 fame point. 



He fuppofed fquinting, alfo, fometimes to be owing to 

 the oblique fituation of the cryltalline humour in one of the 

 eyes. Thefe hypothefes are examined and refuted by Dr. 

 Jurin, who, in order to account for this defcft, confiders 

 the difpofition and fituation of the eyes in a found ilate. 

 When we look direftly forwards at a diltant objeft, the 

 pupil of each eye lies in the middle of the aperture formed 

 by the eye-lids ; fo that the diftance between the two pupils 

 confitts of the breadth of the nofe and half the breadth of 

 the aperture of each eye ; and this diltance between them is 

 always prelerved, liowever obliquely we turn our eyes. In 

 looking at near objefts, the diltance between the pupils is 

 fomewhat lefs, but (till it is always the fame in all oblique 

 direftions of the eyes, as well as when we look Itraight for- 

 wards ; by which means the axes of both eyes are direfted 

 forwards to the fame point, in both thtfe cafes of looking 

 at diltant and near objefts. But in thole who fquint, when 

 the pupil of the undillortcd eye is fcated in the middle of 

 the aperture, as in looking dircftly forwards, the pupil of 

 the other eye is drawn dole to the nofe, and confequently, 

 the diltance between the two pupils ii confiderably lefs than 

 in other perfons ; and this lefs diltance continues the lame 

 in all oblique dircftion'; for the eyes ; fo that two axes are 

 never pointed at the fame objeft, though the mufclcs do fo 

 far aft in concert with each other, as to move both eyes the 

 fame way at the fame inltant of time. 



This vicious habit, Dr. Jurin obfervcs, may eafily be 



contrafted by a child, if he is often laid into his cradle in 

 fuch a pofition as to be able to fee either the light, or any- 

 other remarkable objeft, with one eye only : and when by 

 this means he is brought to fquint, and the habit is con- 

 firmed, he apprehends it will be in vain to attempt a cure 

 by his wearing tubes, or Ihells, with fmall holes to look 

 through ; for, notwithllanding every help of this kind, he 

 will contmue to fee through them dillinftly with one eye 

 only, and will ilill diftort the other. The true method of 

 cure he takes to be the following : when the child is ar- 

 rived at fuch an age as to be capable of obferving direftions, 

 place him, fays he, direftly before you, and let him clofe 

 the undillorted eye, and look at you with the other. When 

 you find the axis of this eye fixed direftly upon you, bid 

 him endeavour to keep it in that fituation, and open hi» 

 other eye. You will now fee the dillorted eye turn away 

 from you towards his nofe, and the axis of the other eye 

 will be pointed towards you. But with patience and re- 

 peated trials, he will, by degrees, be able to keep his dif- 

 torted eye fixed upon you, at leatt for fome little time, after 

 the other is opened ; and when you have brought him to 

 keep the axes of both eyes fixed upon you, as you itand 

 direftly before him, it will be time to cliange his pofture, 

 and to fet him firft a little on one fide of you, and then to 

 the other, and fo to praftife the lame thing ; and when in 

 all thefe fituations he can perfeftly and readily turn the axes 

 of both eyes towards you, the cure is effefted. An adult 

 perfon, he fays, may praftife all this by a glafs, without a 

 direftor, though not fo eafily as with one ; but the older he 

 is, the more patience will be neceflary. Smith's Optics, 

 Rem. p. 30. 



M. Buffou infers, from a great number of obfervations^ 

 that the true and original caufe of fquinting is an inequality 

 in the goodnefs, or in the limits of dillinft vifion in the two 

 eyes. When one of theeyes, he fays, is much weaker than 

 the other, we do not direft it towards the objeft, but make 

 ufe of the (Ironger eye only : when the eyes are equally 

 good, we fee more dillinftly with both eyes than with one, 

 by about a thirteenth part ; but when they are unequal in 

 goodnefs, objefts appear lefs diftinft with both eyes than 

 with one. It is no wonder, therefore, that perfons, fubjeft 

 to this inequality, chufe to make ufe of one eye, and turn 

 the other afidc. This inequality in the goodnefs of the 

 eyes is the general caufe of fquinting, according to M. 

 Buffon ; and other.i!, with refpeft to this, are accidental. 

 All the perfons whom he examined, if they fquinted at all, 

 did fo from this caufe ; and this eiTeft is fo necellary, that, 

 he fays, it is not poffible to cure a perfon of fquinting, 

 whofe eyes diffi-r much in point of goodnefs, except, per- 

 haps, by firll bringing them nearer to an equality by means 

 of glalTes. To the lame purpofe Dr. Reid obfcrves, that, 

 having examined above twenty perfons who fquinted, he 

 found in all of them a defeft in the fight of one eye. And 

 this writer is of opinion, that the centres of the two eyes of 

 fquinting perfons correfpond with them as with others, fo 

 that if they could be brought to the li.ibit of dircftini; iheir 

 eyes aright to an objeft, they would not only remove a de- 

 formity, but likewile impri>ve their light. 



In examining the limits of diltinft vfion in feveral chil- 

 dren who did not fquint, M. BiilTou found, (contrary to 

 Dr. Jurin's obfervation,) that t' '-y did not fee by any means 

 either fo far off, or fo near as .idiilt oerfons ; lo that, as we 

 advance in age, the limits of dillinft vifion are extended 

 both ways; and this, he fays, is one reafon why ciuldrcn 

 fquint mor-- than adult perfons. 



When tlie eyes of ihofe perfons who fquint do not differ 

 much in point of gooducfd, M. Buffon thought that the 



mod 



