The “rods and cones” are lying on a layer of colored or pigment cells whose duty it is to prevent diffusion of light within the eye. The eyeball, therefore, is to all intents a camera obscura, the iris representing the shutter, the crystalline lens the camera lens, and the layer of “rods and cones” the sensitive plate. When a ray of light falls on the layer of the “rods and cones,” this layer receives a nervous stimulus which is conveyed by the optic nerve to the brain. It is these sensations which the brain translates into what we term sight.

Where the optic nerve enters the back of the eye, there are no “rods and cones,” hence rays of light falling on this portion of the retina send no stimulus to the brain; in other words, images falling on the “blind spot” are not visible.

The “yellow spot” is a small area at the center at the back of the eye where the retina is very thin, consisting of little more than a single layer of “cones.” Images which fall upon this region are seen with the greatest distinctness.

HOW THE SENSE OF SIGHT
IS PRODUCED

Sight is a nervous sensation due to the translation by the brain of the effects caused by rays of light being reflected from some object in front of the eye on to the innermost layer of the eye, the retina.

When an object is looked at, rays of light which reach the object from some source of light (such as the sun, a lamp, etc.) fall on the transparent outer part of the eye, the cornea. On account of its curved surface these rays of light are more or less bent inward so as to fall more or less perpendicularly on the forward anterior convex surface of the lens. If the light is weak or dim, the iris, which lies in front of the lens, will automatically contract down so as to make the opening by which the rays can enter the posterior chamber of the eye (the part behind the lens) as large as possible.

If the light is very bright the muscle fibers in the iris will relax so that the iris itself gets larger, and its central opening smaller, so that too much light may not enter. Passing through the lens the rays are focused by the lens so that they are brought together to a point exactly on the surface of the retina.

Here their presence has a certain effect on the rod and cone layer of the retina, the result of which is conducted along the optic nerve to the brain, where it is transformed into what we know as sight.

HOW WE ARE ABLE TO TASTE, SMELL AND FEEL