VISION. 



vifion is performed by the intermiflion of fomething- into the 

 eye, and not by vifual rays, proceeding from the eye, 'i 

 had been the general opinion before his time ; and he was 

 the firft who fully fatisfied himfeif and others upon this fiib- 

 jeA, though feveral philofophers ftill adhered to the old 

 opinion. 



As for the Peripatetic feries or chain of images, it is a 

 mere chimsra ; and Ariftotle's meaning is better underftood 

 without than with them. In effeft, fetting thefe afide, the 

 Ariftotelian, Cartelian, and Newtonian doftrines of vifion 

 are very confiftent ; for fir Ifaac Newton imagines, that 

 vifion is performed chiefly by the vibrations of a fine me- 

 dium, wliich penetrates all bodies excited in the bottom of 

 the eye by'the rays of light, and propagated through the 

 capillaments of the optic nerves, to the fenforiura. And 

 Defcartes maintains, that the fun preffing the materia fub- 

 tilis, with which the world is filled every way, the vibrations 

 aud pulfes of this matter refiefted from objefts are communi- 

 cated to the eye, and thence to the feiifory ; fo that the 

 action or vibration of a medium is equally fuppofed in all. 



Vision, Modern Theory of. In order to vifion, we are 

 certain, it is required, that the rays of light be thrown from 

 tlie vifible objefts to the eye. What befalls them in the eye 

 will be conceived from what follows. 



Suppofe, e. gr. Z the eye, and ABC the object 

 (P/a^i'XX. Optics, Jig. 8.); now, though every point of 

 an objeft be a r.idiant point, that is, though there be rays 

 re.lefted from every point of the objeft to every point of the 

 circumambient fpace, each carrying with it its refpeftive 

 colour, (which wc falfely imagine to be thofe of the objeft,) 

 yet, as only thofe rays which pafs through the pupil of the 

 eye affeft the fenfe, we fhaU here confider none elfe but 

 thefe. 



And again, though there be a great number of rays 

 paffing from one radiant point, as B, through the pupil ; 

 yet we (hall only confider the aftion of a few of them, as 

 BD, BE, BF. 



Now, then, the ray B D, falling perpendicularly on the 

 furface E D P', will pafs out of the air into the aqueous 

 humour, without any refraftion, and proceed right to H, 

 where, falling perpendicularly on the furface of the cryftal- 

 line humour, it will go on, without any refraftion, to M ; 

 where, again falhng perpendicularly on the furface of tke 

 vitreous humour, it will proceed ftraight to the point O, in 

 the fund or bottom of the eye. 



Again, the ray B E, pafling obhquely out of the air 

 upon the furface of the watery humour E D F, will be re- 

 frafted, and approach towards the perpendicular E P ; and 

 thus, proceeding to the point G, in tlie furface of the cryf- 

 talline, it will be there refrafted ftill nearer to the perpendi- 

 cular. So alfo E G, falling obliquely out of air into an 

 harder body, will be refrafted towards the perpendicular 

 G R, and, falling on the point L of the furface of the 

 vitreous humour, it will IHU be brought nearer to M. 



Laftly, G L, falling obliquely out of a denfer, upon the 

 furface of a rarer body L M N, will be refrafted, and re- 

 cede from the perpendicular L T ; in receding from which, 

 it is evident, it approaches towards the ray B D O, and 

 may be fo refrafted, as to meet the other in O. In hke 

 manner, the ray B F, being refrafted in B, will turn to I, 

 and thence to N, and thence to the others in O. But the 

 rays between B E and B F, being fomewhat lefs refrafted, 

 will not meet precifely in the fame point O. 



Thus will the radiant point B affeft the fund of the eye, 

 in the fame manner as if the pupil had no breadth, or as if 

 the radiant itfelf had only emitted one fingle ray, fuch as 

 yere equal in power to all thofe between B E and B F. 



In hke manner, the raye proceeding from the point A, 

 will be fo refrafted in paffing through the humours of the 

 eye, as to meet near the point X ; and the rays from airy- 

 intermediate point between A and B, will nearly meet in 

 fome other point in the fund of the eye between X and O. 



Upon the whole, it may be afierted univerfally, that 

 every point of an objeft affefts one point in the fund of thr- 

 eye ; and, on the contrary, that every point in the fund of 

 the eye only receives rays from one point of the objeft. 

 Though this is not to be underftood with the utmoft rigour. 



Now, if the objeft recede from the eye, in fuch manner 

 as that the radiant point B does not dechne from the hne 

 B D ; the rays which would proceed from B, not enough 

 divaricated, would be fo refrafted in pafling the three fur- 

 faces, as that they would meet before they reached the 

 point O ; on the contrary, if the objeft Ihould be brought 

 nearer the eye, the rays paffing from the point to the pupil, 

 being too much divaricated, would be refrafted fo, as not 

 to meet till beyond the point O : nay, the objeft may be fo 

 near, that the rays proceeding Irom any point may be fo 

 divaricated, as that they fliall never meet at all. In all 

 which cafes, there would be no point of the objeft but would 

 move a pretty large portion of the fund of the eye ; and 

 thus the aftion of each point would be confounded with that 

 of thL^ contiguous one. % 



And this would commonly be the cafe, but that nature 

 has provided againft it ; either by contriving the eye fo that 

 its bulk may be lengthened, or (hortened, as objefts may be 

 more or lefs diftant ; or, as others will have it, fo as that the 

 cryftalline may be made more convex, or more flat ; or, ac- 

 cording to others, fo as that the diftance between the cryf- 

 talline and the retina may be lengthened or fliortened. 



The firft expedient has been thought by fome to be the 

 moft probable ; on the footing of which, when we direft 

 our eyes to an objeft fo remote, as that it cannot be dif- 

 tinftly viewed by the eye in its accuftomed figure, the eye 

 is drawn back into a flatter figure, by the contraftion of 

 four mufcles ; by which means the retina, becoming nearer 

 the cryftalline humour, receives the rays fooner ; and, on the 

 other hand, when we view an objeft too near, the eye, 

 being comprefled by the two oblique mufcles, is rendered 

 more globular ; by which means the retina, being fet farther 

 off from the cryftaUine, does not receive the rays of any 

 point before they meet. See Visible. 



Thofe who maintain the opinion now ftated farther allege, 

 that this accefs and recefs of the cryftalUne is fo neceflary to 

 vifion, that whereas, in fome birds, the coats of the eye are 

 of fuch a bony confiftence, that mufcles would not have 

 been able to contraft and diftend them ; nature has taken 

 other means, by binding the cryftalline down to the retina, 

 with a kind of blackilh threads not found in the eyes of 

 other animals. Nor mU)ft it be omitted, that of the three 

 refraftions above-mentioned, the firft is wanting in fifties ; 

 and that, to remedy this, their cryftalline is not lenticular, 

 as in other animals, but globular. Laftly, fince the eyes of 

 old people are generally worn flatter than thofe of young 

 ones, fo that the rays from any point fall on the retina be- 

 fore they become collefted into one, they muft exhibit the 

 objeft fomewhat confufedly ; nor can fuch eyes fee any but 

 remote objefts diftinftly. In others, whofe eyes are too 

 globular, the cafe is juft the reverfe. See Presbyta and 

 Myops. 



From what has been ftiewn, that every point of an obje6k 

 moves only one point of the bottom of the eye ; and, on the 

 contrary, that every point in the fund of the eye only re- 

 ceives ray* from one point of the objeft, it is eafy to coa- 

 ceive, that the whole objeft movea a certain part of the 



redna; 



