REFRACTION 219 



to enlarge the pupillary opening. Thus the amount of light 

 entering the eye can be regulated. At the back of the eyeball, 

 slightly internal to and just below the antero-posterior axis, 

 the optic nerve pierces the sclerotic and choroid, and its fibres 

 spread out on the inner surface of the latter to form the retina 

 which extends forward almost to the ciliary body (see below). 

 Where the antero-posterior axis meets the retina is a small yellow 

 area, the macula lutea, in the centre of which is a depression, 

 the fovea centralis. The macula constitutes the area of distinct 

 vision. The retina is a sensitive screen, on which a picture of 

 external objects is brought to a focus, and corresponds to the 

 plate of a photographic camera. Along with the optic nerve the 

 arteria centralis retinae enters, and its branches radiate outwards 

 on the inner surface of the retina. Internal to the retina is a 

 delicate layer, the hyaloid membrane, which, like the choroid, 

 becomes free near the corneo-sclerotic junction and splits to 

 enclose the crystalline lens. The latter is a laminated biconvex 

 structure placed just behind the iris, and centred with the pupil. 

 Between the wall of the eyeball and the lens the hyaloid membrane 

 is considerably thickened, and forms the suspensory ligament or 

 Zonule of Zinn. Close to the corneo-sclerotic junction the choroid 

 is thickened and raised in radiating ridges, the ciliary processes. 

 To these the hyaloid membrane is firmly attached, and into them 

 are inserted fibres of the ciliary muscle. The ciliary muscle arises 

 from the corneo-sclerotic junction and its fibres are arranged in 

 two sets a radial passing back into the ciliary processes and a 

 circular. The ciliary processes, plus the ciliary muscle fibres, 

 constitute the ciliary body. The lens, with its attachments, 

 separates the eyeball into an anterior chamber filled with clear 

 fluid, the aqueous humour, and a posterior containing a rather 

 more jelly-like substance, the vitreous humour. The anterior 

 chamber is divided into two compartments by the iris, but there 

 is free communication between them through the pupil. 



Refraction in the Eye. 



Let us now consider the passage of a ray of light through the 

 instrument we have described. We must first know the indices 

 of refraction of the various media. They have been determined 

 experimentally, and may be given approximately as follows : 



Cornea 1-33 



Aqueous - 1-33 



Lens - - 1-43 



Vitreous -------- 1-33 



