Fig. 24.18 is a simplified diagram of the human eye. In many respects it is similar to the camera. It has a tough, white wall called the sclerotic of which the front portion, the cornea, is transparent. Situated in the aqueous humour in front of the eye lens is the iris orcoloured part of the eye which automatically adjusts the size of the pupil or circular opening in the centre according to the intensity of the light falling on it. Light passing through the eye lens crosses the vitreous humour to form an image on the retina. From here, electrical signals are transmitted to the brain via the optic nerve. The main refraction of the light entering the eye occurs at the cornea which has rather more than twice the power of the eye lens. The sharpness of the image formed on the retina for objects at different distances from the eye is controlled by an alteration in the focal length of the eye lens. This is called accomodation and is brought about by the ciliary muscles which vary the thickness and curvature, and consequently the focal length, of the eye lens. In this respect the eye differs from the camera, since, as we have already seen, focussing in the camera is brought about by varying the distance of a fixed focus lens from the film.