S80 THE EYE. [Lesson xxxvn. 



rent fluid, like water, which is therefore called the 

 aqueous humour : it gives a protuberant figure to 

 the cernea, and has the same limpidity and refrac- 

 tive power as water. At the back of this lies a 

 humour transparent like crystal, and much of the 

 consistenca of hard jelly : it is shaped like a 

 double convex glass, and is a little more convex 

 on the back than on the fore part : it is named the 

 crystalline humour) and is of service in converging 

 the rays which pass through it, to its focus at the 

 bottom of the Eye. This humour is enclosed in a 

 fine transparent membrane from which proceed ra- 

 dial fibres, called the ligamentum ciliare^ all around 

 its edge, joining to the circumference of the im. 

 These fibres have a power of dilating and contract- 

 ing occasionally, by which means the convexity 

 of the crystalline humour is altered ; and it is also 

 thereby shifted a little backward or forward in the 

 Eye, so as to adapt its focal distance at the bottom 

 of the Eye to the different distances of objects : .a 

 provision without which we could only see objects 

 distinctly at one particular distance from the Eye.. 

 At the back of the crystalline, lies the vitreous 

 humoitr, which is transparent like glass, as its name 

 denotes, and is largest of all in quantity ; filling 

 the rest of the Eye, and giving it a globular shape : 

 h is much of the consistence of the white of an 

 egg, its refractive power very little exceeding that 

 of water. 



As rays are emitted or reflected from every point 

 of an object; some of these from the side next ihe 

 Eye will fait upon ihe cornea, and, by passing on 



through 



