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The eyes come in two flavors: with a round pupil or with a slit. This is just a small detail because
they both work the same way and they both have much in common with a standard camera. The light first pass through the cornea,
the main source of refraction, then it pass through the lens which controls 1/3 of the refraction of light that enters the
eye. Located just behind the pupil it allows for changing of focus from distance to near objects by altering its shape. This
changing focus is called accommodation. As a person ages the lens hardens and accommodation becomes more difficult. Finally,
the lights reach the retina that contains photoreceptors (rods and cones) that change light into sight by converting light
into electrical impulses. These electrical messages are sent from the retina to the brain and interpreted as images.
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The iris is the colored part of the eye has very fine muscles to control the size of the pupil and thus
the intensity of the light. Being able to reduce the pupils to slits rather than tiny circles gives the creature a greater
and more accurate control of how much light enters their eyes; this ability is particularly important in bright sunlight.
Vertical slits also have an advantage over horizontal slits. When the dragon's eyelids close at right angles to the vertical
pupil, he can reduce the amount of light even further by bringing its eyelids closer and closer together. This combination
of the vertical slits of the pupils and the horizontal slits of the eyelids, allows the creature to make the most delicate
adjustments of the light reaching its eye compared to any other animal.
Also, many animals also have an additional structure called the tapetum - a reflective layer that lies under
the retina and acts as a mirror, bouncing light back toward the retina a second time. Dogs, cats, horses, and cattle all possess
them. This allows them to function under dim lighting better than humans. Cats, for instance, are five times better at detecting
light than humans.
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