on, the eye has Both the lens and the iris are part of the ciliary body whose the facility of automatic diaphragm and automatic focusing control. muscles control the lens curvature and the size of the pupil. The space at the back of the lens and the ciliary body is filled with vitreous humour of refractive index 1.337. Within the sclera is the Structure of the eye choroid which contains an opaque pigment and which is well supplied with blood vessels. This is the absorber of stray light. The Figure 7.12 illustrates the structure of the human eye. It is an almost retina covers much of the surface of the choroid behind the ciliary spherical organ with the viscous mass inside held in place by a tough body and contains the first co-ordinating nerve cells of the visual outer shell, the sclera. The sclera is a whitish colour and is opaque. system. At the back of the retina is a thin layer of light sensitive However, it has a transparent front area which protrudes with a photoreceptor cells known as rods and cones under a protective greater curvature. This is the cornea. The surface of the cornea pigment. Millions of rod and cone cells are intermingled non-uni- provides the first and strongest element of the lens system of the eye. formly over the retina. The rods contain rhodopsin which is This is because the greatest change in refractive index occurs at this bleached by light. In turn the products formed by this bleaching surface; the refractive index of air = 1.000 and the refractive index stimulate nerve conduction. Rods are extremely sensitive and per- 7/10 Optics and vision EXTERNAL MUSCLE LIMBUS SCLERA CHOROLD ANTERIOR CHAMBER RETINA CORNEA MACULA PUPIL FOVEA POSTERIOR CHAMBER CANAL OF SCHLEMM OPTIC NERVE CILIARY BODY CONJUNCTIVA VITREOUS INTERNAL MUSCLE Figure 7.12 Horizontal section of right eye form in light too dim for the cones to operate. This is the region of rely only on the information it receives from one eye but analyses scotopic vision. As an ensemble they are unable to distinguish the images formed si