Gonioscopy describes the use of a goniolens in conjunction with a slit lamp to gain a view of the anatomical angle formed between the eye's cornea and iris. The importance of this process is in diagnosing and monitoring various eye conditions associated with glaucoma. The goniolens allows the optometrist to view the angle through a mirror or prism, without which the angle is impossible to see. The aqueous fluid produced within the eye normally exits through drainage structures in the angle. The goniolens allows the optometrist to check whether these drainage channels are operating effectively. The use of the goniolens requires topical anaesthesia to numb the cornea and is hence a very comfortable procedure.