VISIBLE. 



mcnts, in which an objetl was viewed through fmall flits 

 in a thin plate of iron, at a lefs diitance than the diameter 

 of the papil (which, therefore, was of no ufc in this cafe), 

 tJiat we are poffefTed of a power of changirj^ the conform- 

 ation of our eyes, and of adapting them to various dif- 

 tauces ; and that this change always follows a fimilar 

 motion in the axes of vifion, with which it has been con- 

 netted by ufe and cuitom. Porteriield on the Eye, vol. i. 

 p. 411. 415. 421. 



However, among thofe who fuppofc a conformation of 

 the eye for tiiis purpofe, independent of a variation in the 

 aperture, it is by no means agreed in what it confills. 

 Some have faid, that the cryftalline becomes more or lefs 

 convex for this purpofe, by the adlion of certain mufcular 

 fibres which enter into its compofition. But Dr. Porter- 

 field (ubi fupra, p. 442.) obferves that, though the cryf- 

 talline, when dry, appears to confift of many thin concen- 

 tric laminae, or fcales, their difpofition is but ill qualified for 

 changing the figure of the cryftaUine ; or if they were fo, 

 it is not eafy, he fays, to prove that thefe fibres are mufcu- 

 lar, and capable of contraftion. 



His own opinion is, that the cryftalline has a motion by 

 means of the ligamentum ciliare, by which the diitance 

 between it and the retina is increafed or diminiflied, ac- 

 cording to the different diftances of objefts. The ftruc- 

 ture and difpofition of the ligamentum ciliare, he fays, ex- 

 cellently qualify it for changing the fituation of the cryf- 

 talline, and removing it to a greater diftance from the 

 retina, when objefts are too near for us ; becaufe, when it 

 contrafts, it will not only draw the cryftalline forward, but 

 alfo comprefs the vitreous humour lying behind it, fo that 

 it muft prefs upon the cryftalline, and pu(h it towards the 

 retina. 



He adds, that the cryftalline, being moved forwards, 

 muft, at the fame time, prefs the aqueous humour againft 

 the cornea ; by which means that membrane, which is 

 flexible, will be rendered more convex, and enable us ftill 

 better to fee near objefts diftinftly. 



That the fituation of the cryftalhne is made ufe of in 

 conforming the eye to the diftinft view of objefts placed 

 at different diftances. Dr. Porterfield thinks, is very evident 

 from what is obferved concerning perfons who have ca- 

 taraifts couched ; for the fame lens is not ufeful to them for 

 feeing all objefts diftinftly, but they are obliged to make 

 ufe of glaftt's of different degrees of convexity, in propor- 

 tion to the nearnefs of the objeft. 



To the objeftion of M. de la Hire, and others, among 

 whom are the celebrated anatomifts Hallcr and Zinn, 

 that the ciliary ligament is not mufcular, and confequently 

 has no power of contraftion, he obferves, that they have 

 been led into tliis miftake by apprehending that the colour 

 of mufcles is always red ; whereas this is not the cafe uni- 

 verfally, for the mufcular fibres of the intellines and ftomach 

 have hardly any rednefs in their colour. It is alfo cer- 

 tain, he fays, that the pupil contrafts and dilates itfelf ac- 

 cording as objefts are more or lefs luminous, and yet none 

 of the fibres which perform thafc aftion are in the Icaft red. 

 Ubi fupra, vol. ii. p. 434. 447. 450. 



Dr. .Turin (Etf. on diftinft, &c. Vifion, p. 143.) fup- 

 pofcs, tliat when the eye is to be fuitcd to greater dif- 

 tances than fifteen or fixteen inches, the ligamentum ciliare 

 contrafts, fo as to draw part of the anterior furface of the 

 capfula of the cryftalline, into which the fibres of it are 

 inferted, a little forwards and outwards, on which the water 

 within the capfula muft flow from under the middle towards 

 the elevated part of it ; and the aqueous humour muft flow 

 from above the elevated part of the cnjfula to the middle. 



In confequence of this, the vrhol* anterior furface, within 

 the infertion of the ciliary ligament, will be reduced to a 

 lefs convexity. When this contraftion ceafes, the capfula 

 will return to its former fituation, by its own elafticity. 

 To this hypothefis it has been objcfted, that unlefs the 

 water within the capfula has a greater refraftive power 

 than the aqueous humour, the retiring of it from one place 

 to another to make room for that humour, will have no 

 effeft upon the pencils of rays. 



Dr. Jurin, however, not attending to this circumftance, 

 and feeming to confider the water within the capfula as 

 having the fame refractive power with the cryftalline itfelf, 

 attempts to fhew by calculation, that this change in the 

 convexity of it is quite fufficient to extend the natural 

 diftance of diftinft vifion from fifteen inches to fourteen 

 feet five inches, without the leaft motion of the cryftalline 

 itfelf, and a very fmall one of the anterior furface of the 

 capfula. 



M. Mufchenbroeck, or rather Albinus { whofe Anatomical 

 Obfervations on the Eye he has publifiied in Iiis Introd. ad 

 Phil. Nat. vol. ii. p. 759.), fuppofes, that the change of con- 

 formation in the eye is performed by means of the zona cili- 

 aris, in the following manner. In viewing a very near ob- 

 jeft, in confequence of which the pencils of rays tend to a 

 focus beyond the retina, the zona ciliaris, and the anterior 

 membrane of the capfula, as alfo the vitreous humour, being 

 driven forward by the comprellion of the coats of the eye, 

 pufh the cryftaUine, and make it recede from the retina. At 

 the fame time the cryftalline, pufhing the aqueous humour 

 into the cornea, makes it more prominent. Perhaps, alfo, 

 he fays, the cryftalline may be made rounder, fo that, on 

 thefe accounts, the pencils vHU come to their foci fooner 

 than otherwife. On the other hand, when the objeft is too 

 remote for diftinft vifion, fo that the pencils come to their 

 foci too foon, the zona ciliaris becomes tenfe, and, with the 

 anterior membrane of the capfula, pufhes the cryftalline far- 

 ther within the vitreous humour. By this prefTure the cryf- 

 talline becomes flatter, fo that, on thefe feveral accounts, 

 the foci of the pencils are carried farther. The zona cili- 

 aris, and the anterior membrane of the capfula, can only 

 pufh the cryftalline into the vitreous humour one half of its 

 own thicknefs, which he fhews is not fufiicient to make vifion 

 diftinft at a competent diftance, and therefore concludes, 

 that fome change mufl take place in the form of the cryf- 

 talline, as, he fays, Dr. Pemberton has well demonftrated. 

 He fuppofes, that the provifion for fuiting the eye to dif- 

 ferent diftances is the fame in all animals, and does not de- 

 pend on the change of the fclerotica in any of them, which is 

 hard, and incapable of being comprefTed. Prieftley's Hift. 

 of Light, &c. p. 638 — 652. See y^yY"""'"' Distance. See 

 alfo Eve. 



It feems to be now pretty generally allowed, that the 

 change, by which the eye accommodates itfelf to different 

 diftances, is produced by an increafe of the convexity of the 

 cryftalline lens, arifing from an internal caufe. The argu- 

 ments in favour of this conclufion are of two kinds ; fome 

 of them are negative, derived from the inipolTibility of ima- 

 gining any other mode of performing the accommodation, 

 without cxceediing the hmits of the aftual dimenfions of 

 the eye, and from the examination of the eye in its dif- 

 ferent ftates by feveral tefts, capable of dclefting any 

 other changes if they had exifted : for example, by the 

 application of water to the cornea, which completely re- 

 moves the effeft of its convexity, without impairing the 

 power of altering the focus, and by holding the whole eye, 

 when turned inwards, in fuch a manner as to render any ma- 

 terial alteration of its length utterly injpofTible. Other ar- 



L 1 i gumcnts 



