The retina is the "film" of the eye and vision system. All images are projected on retina in a upsidedown mode caused by the cornea and lens. It is composed of three layers of nerve cell bodies and two layers of synapses. The outer nuclear layer contains cell bodies of the rods and cones, the inner nuclear layer contains cell bodies of the bipolar, horizontal and amacrine cells and the ganglion cell layer contains cell bodies of ganglion cells and displaced amacrine cells. Dividing these nerve cell layers are two neuropils where synaptic contacts occur. The cones are the color receptors of the eye and the rod cells while not sensitive to color, are very sensitive to light at low levels.
There are three types of cone cells, each of which contains a visual pigment.
These pigments are called the red, blue or green visual pigment.
The cone cells detect the primary colors, and the brain mixes these colors in seemingly infinitely variable proportions so that we can perceive a wide range of colors, a common term for this is photopic conditions.
Prolonged exposure to colors, for example when staring at a particular object, can cause fatigue in cone cells.
This results in a change in the way that you perceive the color of the object that you are viewing.
Rod photoreceptors and rod-connected nerve cells through the retina are responsible for pathways concerned with night vision and increased sensitivity of our visual system under what is called scotopic conditions (conditions of very little ambient light).
The rods and cones contain chemicals based on beta carotene(that's why you should eat carrots), the natural, unmodified form of vitamin A. As light strikes the photoreceptor, there occurs a chemical reaction which releases an electrical charge. The chemical reaction continues and the molecule is restored to its original condition - until the next impulse of light energy hits and the photoreceptor sends out yet another electrical charge, over and over again, millions of times a second in a few hundred thousand locations on the retina.
In the centre of the retina, there is an area where only cones exist, the yellow spot or macula.
This is the point where the images in the center of visual focus will fall.
This acts as a massive "bias" to the colours that you notice in the scene that you're looking at, and produces the finest resolution of any part of the image that you see.
At one location, called the optic nerve head, the axons from the ganglioncells all over the retina converge to form the optic nerve which carries information from the eye to the rest of the brain. At this location, there are no photoreceptors, and hence the brain gets no information from the eye about this particular part of the picture of the world. Because of this, you should have a "blind spot"(actually two, one for each eye). We are never aware of this ``hole'', because higher visual processing centers help us reconstruct a solid world [1][2][6][7][8].