No. It’s not a magic invisibility field inside. It’s a magic object that works only on living “intelligent” creatures. It also doesn’t work the same on different races. Only humans (hobbits are a subtype of human in Tolkiens lore) turn invisible. And because it’s magic it also turns their clothing etc invisible. So either Frodo and his poop is invisible or nothing is.
I don’t know the lore, but what initially happens with the ring? Anything that passes through it vanishes?
No. It’s not a magic invisibility field inside. It’s a magic object that works only on living “intelligent” creatures. It also doesn’t work the same on different races. Only humans (hobbits are a subtype of human in Tolkiens lore) turn invisible. And because it’s magic it also turns their clothing etc invisible. So either Frodo and his poop is invisible or nothing is.