ABSTRACT
A horizontal section through the eyeball is shown in Figure 4.1. Few of the details need tobe considered in this book.The eye is an optical systemwith two lenses.The front of the cornea is the surface with the greatest refractive power. The lens itself has less refractive power, but this is variable because the thickness of the lens can change. Parallel rays falling straight into the eye are focussed onto the centre of the fovea, the centre of the retina.When the lens is made thicker (accommodation, Fig. 4.2), a sharp image of nearobjects is obtained.With age the ability to accommodate
decreases (presbyopia), so that a sharp image of a near object is only possible with the help of reading glasses (Fig. 4.3).
