Our Eyes Explained
The human eye is our window to the world and the complex beauty of everything around us. Being one of the most fascinating aspects of the human body, it is a sense organ that allows vision to occur.

The eye behaves in many ways like a camera – we ‘see’ when the lens of our eye starts to focus on an object, whereby the image is then transmitted to the light sensitive membrane at the back of our eye known as the retina. We now know that neurological impulses are carried from the retina to various parts of the brain. The visual association cortex – which is a disproportionately large area of the brain – then pieces together what we ultimately perceive as a visual scene.

Ancient Greek studies had two main schools of thought about how the human eye ‘sees’. One of them described how rays would be emitted from our eyes, bouncing off objects and the movement of the rays would then allow us to see. Aristotle and his followers believed that something entered our eyes and presented us somehow with a representation of an object – therefore this idea was not so far from what we understand today.

Leonardo Da Vinci had theories about eye surgery

It was Leonardo da Vinci however who first began to grasp the wonderful optical nature of the human eye.

“Now, can you not see that the eye embraces the beauty of the whole world? He draws the cosmos, he advises all the human arts and corrects them, he moves humanity to the diverse parts of the world. This is the king of mathematics, whose knowledge is certain; he has measured the distance and size of the stars; he has found the proper place for the elements; he has predicted the future by means of the stars’ course; he has begotten architecture, perspective, and divine painting.” (Manuscript from the Vatican Library, MS Urb. Lat. 1270.)

Leonardo was fascinated by eyes and this was represented in his sketches and paintings. He believed that the eyes were the gateway to the soul and even examined a skull at one point to see if he could fathom how the eyes could see.

It was not until around the 17th century that the real mechanisms of the eye truly began to be understood. For example at this point it was deduced that the retina (the light sensitive layer of tissue lining the inner surface of the eye) was responsible for transmitting nerve impulses to the brain via the optic nerve. Johannes Kepler from Germany and Renee Descartes from France were two well known physicists of the time that made great strides in understanding how our eyes work.

It was in fact Descartes, in an experiment involving an eye from an ox, who determined that an image was being inverted after being focused onto the retina by the lens of the eye. It was Thomas Young however, at the beginning of the 19th century, who correctly described how the lens focuses images onto the retina. He also was able to demonstrate astigmatism where an improperly curved cornea cannot focus light correctly onto the retina.

Understanding Our Eyes Today

This leads us on to today’s understanding of our eyes. Light is reflected off objects and projected onto our retina by the lens. The retina then detects this light and sends impulses along the optic nerve to our brain where the end result is decoded into what we know as vision.

 

 

Perfect vision (known as emmetropia) is achieved when the lens at the front of the eye projects light rays onto the back of the eye (the retina) in sharp focus, helping us to see images clearly. However, just as with a camera, if the lens is not adjusted properly these images can appear blurred. The extent of this blurring is known as refractive error, consisting of short sight, long sight and astigmatism.

Other conditions include presbyopia (a condition related to age where the eye beings to lose its ability to focus on near objects) and of course cataracts.

Eye Facts

• The human eye can distinguish about 10 million different colours.

• All blue eyed people can be traced back to one person who lived near the Black Sea nearly 10,000 years ago.

•  You ‘see’ with your brain, not your eyes. The eyes function in a similar way to a camera – they capture light and send this data back to the brain.

•  Our eyes are very well developed at birth and the coordination of the two eyes and the clarity of vision (visual acuity) are almost completely developed by the age of two.

0

•  The human eye can see 500 shades of grey.

• The eye is made up of over 200 million working parts.

•  The cornea is the transparent covering of the iris and pupil.