820 THE EYE AND VISION [CH. LVttL 



with the capsule of the lens ; round the edge of the lens the canal 

 left is called the Canal of Petit (fig. 507, p. 811), the membrane itself 

 being the Zonule of Zinn. The hyaloid membrane separates the 

 vitreous from the retina. 



Blood-vessels of the Eyeball. The eye is very richly supplied with blood- 

 vessels. In addition to the conjunctival vessels which are. derived from the palpe- 

 bral and lacriraal arteries, there are at least two other distinct sets of vessels 

 supplying the tunics of the eyeball. 



(1) These are the short and long posterior ciliary arteries which pierce the 

 sclerotic in the posterior half of the eyeball, and the anterior ciliary which enter 

 near the insertions of the recti. These vessels anastomose and form a rich choroidal 

 plexus ; they also supply the iris and ciliary processes, forming a highly vascular 

 circle round the outer margin of the iris and adjoining portion of the sclerotic. The 

 distinctness of these vessels from those of the conjunctiva is well seen in the 

 difference between the bright red of blood-shot eyes (conjunctival congestion), and 

 the pink zone surrounding the cornea which indicates deep-seated ciliary congestion. 



(2) The retinal vessels (fig. 516) are derived from the arteria centralis retinae, 

 which enters the eyeball along the centre of the optic nerve. They ramify all over 

 the retina, in its inner layers. They can be seen by ophthalmoscopic examination. 



The Eye as an Optical Instrument. 



In a photographic camera images of external objects are thrown 

 upon a screen at the back of a box, the interior of which is painted 

 black. In the eye, the camera is represented by the eyeball with its 

 black pigment, the screen by the layer of rods and cones of the retina, 

 and the lens by the refracting media. In the case of the camera, 

 the screen is enabled to receive clear images of objects at different 

 distances, by an apparatus for focussing. The corresponding con- 

 trivance in the eye is called accommodation. 



The iris, which allows more or less light to pass into the eye, 

 corresponds with the diaphragms used in the photographic apparatus. 



The refractive media are the cornea, aqueous humour, crystalline 

 lens, and vitreous humour. The most refraction or bending of the 

 rays of light occurs where they pass from the air into the cornea ; they 

 are again bent slightly in passing through the lens. Alterations in 

 the anterior curvature of the lens lead to accommodation. 



We may first consider the refraction through a transparent 

 spherical surface, separating two media of different density. 



The rays of light which fall upon the surface exactly perpendicu- 

 larly do not suffer refraction, but pass through, cutting the optic 

 axis (0 A, fig. 520), a line which passes exactly through the centre 

 of the surface, at a certain point, the nodal point (fig. 520, N), or 

 centre of curvature. Any rays which do not so strike the curved 

 surface are refracted towards the optic axis. Kays which impinge 

 upon the spherical surface parallel to the optic axis, will meet at a 

 point behind, upon the said axis which is called the chief posterior 

 focus (fig. 520, F-L); and again there is a point on the optic axis in 

 front of the surface, rays of light from which so strike the surface 



