| Human vision is amazingly sophisticated in what it can sense. Here are a few facts about our vision.
1. There is a spot on your retina in each eye where your optic nerve is attached. Your retina is not sensitive to light in this spot. Hence, your brain must make up information to compensate. This is a pretty well known fact. See the writeup in blind spot for more details.
2. Almost everyone has three different molecules in their retina that electronically excite (upon absorbing a photon) at three different wavelengths -- roughly red, green, and blue. We see all the colors of the rainbow as a mixture of these three wavelengths. Color blind people (usually men) see only two.
It has been postulated by certain geneticists that in rare cases, the mother of several color blind boys might contain molecules in her retina that are sensitive to four wavelengths (rather than the standard three). Such a person might be referred to as a tetrachromat. A recent study conducted in the United Kingdom may have successfully identified such a woman. She claims to have a very good memory for colors, and seems to be much better than most people at matching two colors quickly.
3. Our eyes are capable of detecting the polarization of the light incident on them (whether parallel or perpendicular). The characteristic pattern (which our brain usually erases) is yellow and bow-tie shaped, and referred to as Haidinger's brushes. See the writeup of this title for further information. | 
