I'd Name Him Wan Li

I'm allergic to cats, to the point that I'm almost willing to say I'm deathly allergic to them.  That doesn't mean that I don't think they're very cool. At the very least they've got that whole aloof predator thing going.  Plus, cat's have those wicked slit pupils.  Cat's wouldn't be nearly as cool if their eyes didn't form slits when they contracted would they?  I mean, try and picture a cat that had tiny round pupils in bright light.  They'd lose a lot of mystique.

Oh.  Hey there kitty.

Alright, so not all cats have slitted pupils.  As a generalized statement, animals within the feline family are more likely to have slitted pupils if they're small, and the larger ones tend to have round pupils like people do.  The lynx, which is something of a mid-sized feline has pupils that aren't round, but aren't quite slits either.

The reason for slitted pupils is that they reduce chromatic aberration, which is breaking the light into its spectrum colours.  You'll see it a lot with crappy digital cameras.  It's sometimes called fringing, because it leaves fringes of colour along the edges of things, with the fringe getting wider near the edges of the image.  It caused by light refracting at different amounts at different wavelengths, and it's more pronounced when put through a rounded lens (like your eye).  The slitted pupil doesn't eliminate the problem entirely, but it does go a long way.

What's interesting about this is that there doesn't seem to be any logic to why the large cats have round pupils while the small cats have slitted pupils.  One could argue that it has to do with diurnal (daytime) or nocturnal hunting habits, with the nocturnal hunters having slits so they can protect their sensitive eyes during the day (slit pupils can close completely, round pupils can't), but it doesn't hold up fully.  Size seems to be a bigger factor than hunting times.

Even the currently accepted evolutionary tree for cats doesn't help here.  Yes, it splits the panthera genus from the smaller cats very early on, which would explain the pupil thing there, but then we've got ocelots (small, slit pupils) further removed from the modern house-cat than the puma (large, round pupils).  Servals (small, slit pupils) are further removed than cheetahs (large, round pupils).  There are exceptions to the small = slits; large = round rule though (isn't there always an exception?), but it's still the best general rule for pupil shape we've got for felines.

Cat eyes: Beautifully designed, arbitrarily assigned based on size.

That's it for the fun fact of the day, but feel free to keep reading, since I kept writing.

My personal favourite exception to the small = slit pupil rule?  The Pallas's cat.  It's about the size of a house-cat, although the thick fur makes it look quite a bit bigger, it has round pupils, and short legs for a feline.  I would totally get one as a pet if I wasn't nearly deathly allergic to cats.

And if they weren't borderline endangered.

They have a few things stacked against them in the survival game, the first being that they've got great fur.  Until humans came around and started wearing the skins of other animals to stay warm, that was an advantage.  Now, it's a liability.  Better to be ugly with no usable parts than be warm and fuzzy.  Particularly if your habitat includes places that still use powdered tiger penis as folk medicine.

Hunted for his fur.
With good reason.

Another big problem for our short-legged pal the Pallas's cat is that it eats agricultural pests.  That means that people don't hunt them for their fur nearly as much as they used to, because they're useful.  Unfortunately, they're not so good at their job as pest control that the farmers have stopped poisoning the pests.  A dead pika (rock rabbits, not to be mistaken for a pikachu, the electric mouse) is a much easier meal for a Pallas's cat than a live one. Unfortunately, they're usually dead because they ate poison.  Which means the Pallas's cat is poisoned by eating the dead pika. And good luck convincing a farmer to stop laying out the poison that's saving his food supply from pests just so that a wild cat won't die.

Also going against the poor round-eyed Pallas's cat is the fact that it's pretty isolated.  Again, a benefit before people starting hunting them (still something of a benefit in that regard as well), but now it's actually working against them.  Their isolation from, well, pretty much everything has meant that their immune system is almost laughable (it would be if it didn't mean these awesome cats were dieing).  Captive breeding programs have met with limited success because captive animals have a nasty tendency of dropping dead from infections that most other animals would survive, or wouldn't even noticeably contract.

Pallas's cats: A cool little cat with big-cat round pupils.

---

Sources:
Tiger eye image taken from Crystal Springs High School, Mississippi
Chromatic aberration sample image taken from TLC-Systems.com
Pallas cat image taken from ShawnOlson.net
Cat pupil shapes courtesy of dozens of Google image searches
PubMed abstract on pupil shapes and lens optics in terrestrial vertebrate eyes
AllExperts.com article on feline pupil shapes
Niches blog on feline evolution
National Geographic article on feline evolution
Wikipedia articles on Family Felidae, Genus Panthera, cats, chromatic aberration and Pallus's cat