But this third eyelid will also show once a cat is happy. So you will know what mood your cat is in just by looking at the membrane.
Your feline friend's pupil of the eyes should also be observed. A change in size of one pupil can be an indication of an inflammation. It can also be a sign of a kind of a disorder in neurologic called Horner's Syndrome. Or worse, it can also indicate tumors or injuries in the central nervous system.
The Nocturnal Vision
Do not believe what others say that cats are color blind. They see some. They may not see as clear as humans do, but it's not like that they cannot see any colors at all.
Cats can also see clearly from afar. It's very different from people. They find it blurry to be looking at things that are so close to them.
Yes, cats have the nocturnal vision. But they can't see in complete darkness. Do you see the muscles on their iris that surround the pupils of their eyes? They are constructed in a way that it narrows into a vertical split when they are exposed in the bright lights. And it opens fully when the acts are in a quite dim light.
This feature of the cats may be traced back to their roots. They can use such to hunt for food during the night if they were left out in the jungle or left with no one to care for them.
So forget about the horror flicks. You've already got the thrills. Not it's time to put some effort into cat eye care. Do not let a minor thing worsen. Call the vet immediately when something's amiss. You don't want your cat to lose their sight or even lose their lives. It is always better to be safe than blind.