THE EYE AS OPTICAL INSTRUMENT. 515 



is white and opaque, and is called the sclerotic. 

 In front, however, this fibrous capsule of the eye, 

 though it does not change its essential character, 

 becomes transparent, and re- 

 ceives the name of the cornea, 

 a I) in fig. 282, in which the trans- 

 parent portion is indicated by two 

 lines only, while the opaque por- 

 tion is distinguished by a dark 

 shading. The corneal portion of 

 the coating of the eyeball is more 

 convex than the sclerotic portion, FlG ' 282 (i rcal si ^' 

 as is shown in the figure. Within the sclerotic and 

 in close contact with it is a highly vascular mem- 

 brane, the choroid coat, which is covered with a very 

 black velvety pigment ; the choroid coat is indicated 

 in the figure by a rather full black line. In front 

 of the eye the choroid coat passes into the iris, cd, 

 which is a kind of diaphragm consisting of radiating 

 muscular fibres and having a round central aperture 

 (e in the figure) called the c pupil.' The iris and pupil 

 are seen through the cornea ; the former has different 

 colours in different persons. Within the choroid coat, 

 and lining the interior of the eye, is the retina, indicated 

 in the figure by a curve consisting of very short lines ; 

 it is a very delicate membrane, consisting of a network 

 of extremely fine nerve?, all of which proceed from one 

 branch, called the optic nerve (f in the figure), which 

 enters the eye obliquely at the back of the eyeball. 

 Immediately behind the iris and in contact with it lies 

 the crystalline lens, g, which has the form of a bi-con- 

 vex lens of unequal curvature, and is perfectly trans- 



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