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The Human Eye

The eye has a convex lens made of tough gelatinous material.  The lens forms an image of an object on a screen called the retina at the back of the eye.  The retina is covered by a large number of nerve fibers, very sensitive to light.  They carry the impression to the brain through the optic nerve.

The front portion of the eye is called the cornea and is made up of a transparent material.  The pupil is a hole in the middle of the colored diaphragm called iris, and appears black because no light is reflected from it.  The iris adjusts the amount of light entering the eye by altering the size of the pupil.


The ciliary muscles alter the focal length of the lens by changing its thickness.  This enables the eye to adjust itself to objects at different distances. This action of the eye is called its power of accommodation.  In a normal eye, the power of accommodation stops when the object is at a distance of about 25cm from the eye.  If the object is brought closer than 25cm, the image formed on the retina is not sharp and the object appears blurred.  The least distance at which the eye can clearly see an object is called the near point or the least distance of distinct vision.  The far point of the eye is the maximum distance at which the eye can see.  For a normal eye it is at infinity.

Persistence of Vision



So, how does this animation work? What causes a still image to appear to move smoothly across a scene? Are your eyes playing tricks on you? Yes, in a way they are. A simple theory known as persistence of vision offers an explanation. The Greek astronomer Ptolemy discovered this principle back in 130 A.D. The retina captures and holds an image for one-sixteenth of a second before processing the next image. If images are flashed before the eye at at least 16 frames per second, the brain thinks it is seeing a single moving image.

This principle is widely used in cinematography.

Seeing more than your Eye Does

Most people (even many who work on the brain) assume that what you see is pretty much what your eye sees and reports to your brain. In fact, your brain adds very substantially to the report it gets from your eye, so that a lot of what you see is actually "made up" by the brain.

Some special features of the anatomy of the eyeball make it possible to demonstrate this to yourself. The front of the eye acts like a camera lens, differently directing light rays from each point in space so as to create on the back of the eye a picture of the world. The picture falls on a sheet of photoreceptors (red in the diagram), specialized brain cells (neurons) which are excited by light.


The sheet of photoreceptors is much like a sheet of film at the back of a camera. But it has a hole in it. At one location, called the optic nerve head, processes of neurons collect together and pass as a bundle through the photoreceptor sheet to form the optic nerve (the thick black line extending up and to the left in the diagram), 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), a place pretty much in the middle of what you can see where you can't see.


Look around. Do you see a blind spot anywhere? Maybe the blind spot for one eye is at a different place than the blind spot for the other (this is actually true), so you don't notice it because each eye sees what the other doesn't. Close one eye and look around again. Now do you see a blind spot? Maybe its just a little TINY blind spot, so small that you (and your brain) just ignore it. It is actually a big blind spot, as you will see if you look at the diagram below and follow the instructions.





Close your left eye and stare at the plus mark in the diagram with your right eye. Off to the right you should be able to see the spot. Don't LOOK at it; just notice that it is there off to the right (if its not, move farther away from the computer screen; you should be able to see the dot if you're a couple of feet away). Now slowly move toward the computer screen. Keep looking at the cross mark while you move. At a particular distance (probably a foot or so), the spot will disappear (it will reappear again if you move even closer). The spot disappears because it falls on the optic nerve head, the hole in the photoreceptor sheet.

So, as you can see, you have a pretty big blind spot, at least as big as the spot in the diagram. What's particularly interesting though is that you don't SEE it. When the spot disappears you still don't SEE a hole. What you see instead is a continuous white field (remember not to LOOK at it; if you do you'll see the spot instead). What you see is something the brain is making up, since the eye isn't actually telling the brain anything at all about that particular part of the picture.



What is a Cataract?


A cataract is a clouding of the part of your eye called the lens. A cataract is not a growth or a film over the eye. It is very common and is usually caused by the simple process of getting older. Almost everyone over the age of 60 years has some degree of cataract formation in one or both of their eyes. Eye injuries, diabetes, or other diseases and certain medications can contribute to their formation. The only cure for cataracts is surgery




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