Apparently we now know why cats have slit eyes:
Almost all animals with multifocal lenses have slit pupils, which help them to make the most of their unique lens, according to the paper. This is because, even when contracted, a slit pupil lets an animal use the full diameter of the lens, spanning all the concentric refractive zones, allowing for all colours to be sharply focused.
When round pupils, such as those in human eyes, constrict, they cover the outer refractive rings of the lens, preventing the focusing of certain colours.
I used to be creeped out by cat eyes, back when I was more a dog sort of person. I wonder how much of the dislike for cats by some people is due to their alien-looking eyes.
But, once again, they just turn out to be better designed creatures than dogs.