t 3*4 ] [Book IX, 



C H A J>. XLI. 



SIGHT. 



Dejcription of the Eye. Eyes of different Animals. How Vifion is 

 performed. How all the Parts cf an Oljeft are comprehended by the 

 Eye. An Image of every Qbjeft painted on the Retina of each Eje, 

 and yet only one Objtfl perceived. Caufe and Cure of fquinting.-*- 

 T'he Senfe of Sight limited. By what Means we judge of Dijlance. 

 State of the Sight at dijj'erent Ages.Cautiovs for prefer-uing 

 the Sight. 



TH E eyes, thofe exquifite organs which raife the 

 perceptive powers of the mind to fome compa- 

 rifon with thofe of fuperior beings, and which in an 

 inftant of time admit impreflions from an almoft infinite 

 variety of objects, are in their ftruclure extremely 

 fimple. They are fituated in two cavities, the or- 

 bits, which afford them protection from a great variety 

 of external injuries, and contain a quantity of fat, 

 which anfwers the purpofe of a foft cufhion, on which 

 they may reft, and perform their different motions with 

 eale and fafety. The globe of the eye is immediately 

 covered by the eyelids, which are continuations of the 

 common integuments of the body, doubled inwards, 

 and attached to the eye, by which they produce what 

 is called the tunica conjunctiva. Where the two eye- 

 lids are united together, they form the canthi, or 

 angles of the eyes ; that next the nofe is called the in- 

 ternal large or inferior angle ; the other, on the con- 

 trary, which is next the temples, is called the external 

 fmall or fuperior angle. The edges of both eyelids 

 are furnifhed with rims of cartilage called the tarfi ; 

 on the margins of thefe, which are called eiliary edges, 

 are fituated febacious glands, which diicharge an orly 

 fluid for the purpofe of preventing adhefion. The 

 2 ciliary 



