The shape of the pupil in a cats eye is an oblong slit that grows wider in dim light, and can cover the entire area of the iris, which is the colored part of the eye. This odd shape gives the cats eye a different appearance than that of human eyes, which have round pupils in the middle of their irises. This odd shape, coupled with the peculiar behavior fo cats, is the foundation of the cats eye being the protector of the so-called "evil eye". If you ever saw any of the "Lord of the Ring" movies, you will recall the evil lord Sauron could only assume the shape of a burning evil eye, suspended in his tallest tower, until he regained control of the ring that was meant to rule over all of Middle Earth. The shape of this burning eye was similar to that of what I have described as a cats eye, with the oblong pupil running from top to bottom.