Visual blind spot is the part of the eye where the nerve fibres of the retina collect and pass out as the optic nerve. This area, about 1/20 of an inch in diameter, has no visual cells and is therefore blind. Want to find out for yourself? Stare at the X with the left eye closed. Start at about 18″ and slowly bring the page closer. At a certain distance, the image of the face falls on the blind spot and it disappears. It’s called the visual blind spot.
How does the eye judge motion?
Essentially by comparing the stability of two objects. Again, the master controller, the brain has to decide. Suppose you are riding in a train, seeing lamp-posts move is obviously taken as motion. However, seeing another train next to you move with no other clues like vibration or sound, gives an illusion of yourself moving though your train is stationary. Hence the eye is merely the outpost of visual ability. Without the computer—the brain—to judge and interpret, the sense of vision, as we know it, does not exist.
How does the eye see rapid individual movements as in a cinema as a continuous movement?
The eye has the ability to fuse individual “flicker” movements into one homogeneous whole. When a blinking light is presented slowly to the eye, it sees it as “on” and “off” separately. As the speed is increased there is a continuous flicker, but the light stays “on”. As the frequency of the blinking is increased the flicker suddenly vanishes.
This fusion frequency is important. The usual level in bright light is 40 cycles per second for the fovea (the most sensitive part of the retina) to 20 cycles per second in the periphery of the retina. However, it decreases in the dark, especially if one is tired or has taken alcohol, and a flickering light can cause a severe headache and nausea.
The annoying flicker of a fluorescent tube is similar in origin. Normally the flicker is at 60 cycles per second. But if the tube becomes old or defective the flicker rate drops to 35 cycles per second and the flicker then becomes very noticeable and irritating.
Why do we need two eyes?
Two eyes provide a wide field of vision thus permitting a panoramic view. They also have the advantage of providing perception of depth or stereopsis. The perception of depth is present only in the area where the fields of both eyes overlap. (Try threading a needle from the corner of the eye. You will find it extremely difficult if not impossible.)
The two eyes, working together, increase the ability to assess and judge details of objects with greater clarity.
Also since the seeing part of one eye matches with the blind spot of the other, with two eyes working there are no dangerous voids or blank spaces in vision.