multaneously on the retina of two eyes. Since colour and the images relayed are not well defined. the two eyes are in different positions the image received by one is There is a smaller population of cones than rods. The cones slightly different from the image received by the other. By this contain isodopsin and respond in bright light giving detailed col- means the degree of parallax occurring between objects at different oured images. This is the region of photopic vision. The optic nerve distances is measured by the brain. This enables it to conceive the is spread over the back of the area of the retina. Signals from the objects in three dimensions and to estimate the distance of the retina are then brought out via the optic nerve. The point of exit objects. contains no receptors and is therefore insensitive to light. This is known as the blind spot. Almost in the centre of the retina is a small depression known as The eye as an optical instrument the macula which is around 2.5mm to 3mm in diameter. In the centre of the macula there is an area of about 1.5mm in diameter called the 7.8.1 Resolving power fovea. In turn in the centre of the fovea there is a tiny area of about 0.5mm in diameter where there are no rods. In this region the cones The rods and cones of the retina of the eye give it a mosaic structure are longer and thinner and more densely packed than cones else- which determines resolution. Minimum resolution depends on the where in the retina. In terms of sensitivity of the retina to light, this retinal location of the image, the nature of the image and the varies depending on the area stimulated. resolution criteria used and adequate time for stimulation. Two The fovea is the most sensitive area and provides the finest detail small dark objects can be resolved as two when their images spread and colour. To take advantage of this an image is constantly shifted over two cones provided the diffraction patterns are sufficiently across different receptor cells by continuous eyeball movement in separated