I've noticed that Templeton's nictating membrane (the internal eyelid) is much more prominent/active than on normal house cats that I've seen. You can just see the membrane on my parents' cats, and it never ever moves. His moves, responds to the sun and drowsiness, and can cover around a third of his eye at times. When he's walking around/playing, you can't really see it, it's only in these special cases that it comes out.

Is this normal on bengals?

All my cats over the years had visible 'third eyelids'..
not while alert etc, but definately when sleepy etc.. or extra purry..
would have thought its normal for all cats..

I've noticed it more in my Bengal kittens than in my previous moggies but it does only seem to be at times when they're resting (in Bengal terms!!) so I haven't been concerned about it.