Eye. Light micrograph showing a longitudinal section of the eye. The eyeball has a tough outer coat, know as the sclera; its front circular part, the cornea (centre right lighter pink), is transparent and protrudes slightly. The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea a shallow chamber is found full of aqueous humour (watery fluid). The large pink feature seen at centre right is the crystalline lens which provides the eye with some additional focusing power. Behind the lens is a large cavity filled with a clear gel. The optic nerve (purple) is seen at lower centre left.