It does, in fact, turn humans invisible too. Isildur being the obvious example, but even the nine rings given to humans had that effect, shifting them to the spiritual / unseen world. That’s a whole different ramble, but for now, let’s sum it up that there is an unseen world not everyone can sense and influence, but the Maiar (including Sauron) are inherently spiritual beings that took physical shape in the seen world in order to interact with it.
For Sauron, so much of his power was poured into the One Ring that he was no longer able to take physical form without it (though he evidently still had some ability to twist minds even without it). Through the Ring, he had also dominated the nine human Ring bearers and bound them to him, moving them into the spiritual world. Given that they were originally of the seen world, they could take physical form more easily than him, but as his power waned, so did theirs and they eventually disappeared until his power grew once more and allowed them to reappear.
The reason they could still “see” Frodo is that they were attuned to the unseen and could sense him there, with their power over it manifesting in them stabbing his physical form even though it was invisible to mortal eyes.
There is still the question of the Dwarven rings. They were forged first, and it’s possible they weren’t as refined yet, though the dwarves are also described as more resilient at resisting the dominating effect. My guess is that the fact they were created by Aulë, Smith of the Valar, rendered them less susceptible to the craft of a lesser spirit (Sauron), but I have no evidence.
Sauron still has a physical form during the events of LOTR. Frodo sees him through the tower window when walking towards Mount Doom, and Gollum remarks he was personally tortured by him, and that his hand has 4 fingers.
Dwarves seem resistant to the rings because of their mechanical nature. As you described, they were first designed by Aule instead of Eru, and then given free will, so that gives them a more “automaton” nature than the other free peoples
It does, in fact, turn humans invisible too. Isildur being the obvious example, but even the nine rings given to humans had that effect, shifting them to the spiritual / unseen world. That’s a whole different ramble, but for now, let’s sum it up that there is an unseen world not everyone can sense and influence, but the Maiar (including Sauron) are inherently spiritual beings that took physical shape in the seen world in order to interact with it.
For Sauron, so much of his power was poured into the One Ring that he was no longer able to take physical form without it (though he evidently still had some ability to twist minds even without it). Through the Ring, he had also dominated the nine human Ring bearers and bound them to him, moving them into the spiritual world. Given that they were originally of the seen world, they could take physical form more easily than him, but as his power waned, so did theirs and they eventually disappeared until his power grew once more and allowed them to reappear.
The reason they could still “see” Frodo is that they were attuned to the unseen and could sense him there, with their power over it manifesting in them stabbing his physical form even though it was invisible to mortal eyes.
There is still the question of the Dwarven rings. They were forged first, and it’s possible they weren’t as refined yet, though the dwarves are also described as more resilient at resisting the dominating effect. My guess is that the fact they were created by Aulë, Smith of the Valar, rendered them less susceptible to the craft of a lesser spirit (Sauron), but I have no evidence.
Sauron still has a physical form during the events of LOTR. Frodo sees him through the tower window when walking towards Mount Doom, and Gollum remarks he was personally tortured by him, and that his hand has 4 fingers.
Dwarves seem resistant to the rings because of their mechanical nature. As you described, they were first designed by Aule instead of Eru, and then given free will, so that gives them a more “automaton” nature than the other free peoples