I was reading another thread here and came across a technical term rheum that I didn't know. I went to wikipedia for more information and saw a picture there where the caption says
Rheum in cat eyes.
Shouldn't it be cat's eyes with the possessive 's? Or either is possible? If so, can you please explain why?
Both are allowed, with subtly different meanings.
"Cat eye" uses cat as a noun adjunct (noun serving as an adjective) to tell you want sort of eye it is.
"Cat's eye" uses the genitive of cat to tell you who or what the eye belongs to.
In the case of the caption, either makes perfect sense.
In other circumstances we might want to use the genitive, particularly if we were referring to a particular cat (that cat's eyes would of course be cat eyes, but we care about the eyes of that cat, not cat eyes in general).
In yet other circumstances we might choose equally between "cat eyes" (talking about the type of eyes cats have) and "cats' eyes" (talking about the same thing, but using the fact that cats have them to specify rather than using cat in an adjectival way).
Some people strongly favour only using the genitive with people, and would therefore prefer not to use "cat's eye", and some (Fowler included) strongly favoured only using it with living things (so he would allow "cat's eye", but not "chair's back"). Personally I think that is nonsense, but the opinion does exist, though less common today than over the last couple of centuries. (Earlier still there was no such opinion, to judge from the KJV and other texts of the time).
All in all though, "cat eye" and "cat's eye" would both be fine in that context. There's the minor advantage that "cat's eyes" has two further meanings (a type of semi-precious stone and a type of road marking), though confusion here is hardly likely.
