Sight is one of the 5 main senses, and is a valuable gift that’s not always fully appreciated by most people. However, it is possible to lose your sight, either because of age-related degeneration, or because of other problems suffered by the human body.
Understanding how the human eye works can help you to understand how to protect your sight. What follows is a brief overview of how the human eye works, followed by a look at a few of the different eyes that can be found in the animal kingdom.
Structure of the Eye
At the front of our eyes sits a transparent structure known as the cornea. The cornea helps to focus light which is coming into the eye. Behind the cornea is the iris, which is the coloured structure that determines your “eye colour”. The iris contains a circular opening known as the pupil, which expands and shrinks to change the quantity of light which is able to enter the eye. 
Between the iris and the cornea there is a thin layer of protective fluid known at the aqueous humor. Muscles in the eye hold and control the lens. When the muscles relax, they cause the lens to flatten, meaning that the eyes focusing power increases. When the muscles contract, they cause the lens to thicken and the fpcusing power of the eye decreases. Light must then pass through another layer of different humour known as vitreous before it reaches the retina at the back of the eye.
The retina consists of millions of cells known as rods and cones. Rods help with monochrome vision in poor light conditions, whereas cones detect fine details and colours. These light sensitive cells turn the light that they pick up into electrical signals, which are carried to the brain by the optic nerve. Once received, the brain instantly translates these signals into images, so that we know what is around us.
Problems
With so many components involved, things can easily go wrong with out eyes. If one of these parts fails to work properly, it can affect the way that a person is able to see. As people age, the cells at the back of the eye start to die off, and do not get replaced. This causes age-related vision problems. Other people develop thick globules in their humour, which can cause spots to develop in their vision. Muscular issues can prevent the ciliary muscles from controlling the lens properly. Regular visits to your optician can help you to treat preventable problems before they become more serious.
Human Eyes vs. Animal Eyes
Not all creatures on Earth are able to see in quite the same way. Some creatures have poorer vision than humans, whereas other animals can see things in far more detail than humans could ever dream of. Other animals may have improved vision in some respects, but not in others. For example, scientists discovered that cats have much better peripheral vision than humans do, however they cannot see in as wide a range of tones and colours. Whereas cats can see up to 8 times better in dark conditions, they do not see as well over distance. This is partially due to the needs of cats, compared to the needs of humans. As natural hunters, cats need to be aware of small changes around them as they stalk their prey. As they spend a lot of time outside after dark, they also need to be able to see well in lower light conditions. Failing to see something in these conditions could either mean that they may get caught by a predator, or they may miss their prey. Therefore, seeing well in these conditions could be a matter of life or death. In terms of biology, these differences occur because of the shape of the retinas and the size of the pupils on each mammal.
Another animal that is adept at seeing in the dark is the Tarsier, which are famous for their exceedingly large eyes. Each eyeball is actually larger than the brain of the animal, and is massive in comparison to the size of the creature. If humans were to have eyes of similar size in comparison to their bodies, their eyes would be as large as grapefruits. They have evolved to be able to pick up the smallest amount of light so that the Tarsier can hunt after dark.
Other animals, such as spiders, have more than two eyes. Spiders have multiple pairs of eyes, and each pair works together to perform a slightly different function. These help the spiders to detect very small amounts of movement. A spider will probably be aware of you before you see it. If it has to come out in the open, it will detect whether you are going to be a threat to it before it makes its move.