CHAP, in.] SIGHT. 839 



Between the iris, which is the extreme front of the choroid 

 investment, and the cornea, which is the extreme front of the 

 sclerotic investment, the lymphatic spaces which over the rest of 

 the eye are narrow and linear are developed into a large con- 

 spicuous chamber, the anterior chamber of the eye, which upon the 

 establishment of the pupil by the absorption of the pupillary 

 membrane becomes continuous with the smaller " posterior cham- 

 ber " of the eye or space between the back surface of the iris and 

 ciliary processes on the outside and the suspensory ligament with 

 the lens on the inside. The cavity of the conjoined anterior and 

 posterior chambers, being a continuation and enlargement of the 

 flatter spaces between the choroid or pia mater of the eye, and 

 sclerotic or dura mater of the eye, may be likened to the sub- 

 arachnoid space, and like that space contains a peculiar fluid ; this, 

 which is called the aqueous humour, like the cerebro-spinal fluid, 

 differs from ordinary lymph, and is probably, to a large extent, 

 furnished by the ciliary processes in some such way as the cerebro- 

 spinal fluid is furnished by the choroid plexuses ( 517). 



The Formation of the Retinal Image. 



526. The iris and choroid coat contain, as we have said, 

 muscular elements, and by means of these muscular elements 

 changes in the form and relations of some of the parts of the eye 

 are brought about ; hence we have to distinguish between the eye 

 at rest, and the eye which is undergoing one or other of these 

 changes. 



The eye is a camera, consisting of a series of surfaces and 

 media arranged in a dark chamber, the iris serving as a diaphragm ; 

 and the object of the apparatus is to form on the retina a distinct 

 image of external objects. That a distinct image is formed on the 

 retina, may be ascertained by removing the sclerotic from the 

 back of an eye, and looking at the hinder surf ace of the transparent 

 retina while rays of light proceeding from an external object are 

 allowed to fall on the cornea. To understand how such an image 

 is formed, we must call to mind a few optical principles. 



A dioptric apparatus in its simplest form consists of two media 

 of different refractive power separated by a (spherical) surface ; 

 and the optical properties of such an apparatus depend upon (1) 

 the degree of curvature of the surface, (2) the relative refractive 

 powers of the media. 



Such a simple optical system is represented in Fig. 140, where 

 apb represents, in section, a curved (spherical) surface separating 

 a less refractive medium, on the left hand towards 0, from a 

 more refractive medium on the right hand towards A. The 

 surface in question is symmetrically placed as regards the line 

 OA, which falling normal (perpendicular) to the surface at p 



