The bond completes; the magic blooms.
Seeing through the mask's eyes is just as disorientingly informative as the Assistant led him to expect.
Physically, he can see more clearly than he ever has in his life, with hints of alien colours accenting the ones he's used to—a glow of heat in his hands that's a red deeper than red, the faint rainbows in the pale silvery mist showing a gleam along their inner edge of a violet beyond violet. Details are sharper, and sharper still if he focuses. His peripheral vision is not narrowed by the mask's occlusion but expanded by its magic; he can't actually see the inside of his mask anymore, and he can see things above and to the sides at a wider angle than before, with a broader range within that angle visible in detail before it starts to blur out at the edges.
Then, overlaid on top or intertwined between the physical aspect of vision, there is also the soulsight. The pile of spirit-scraps on the tree-stump worktable, visible under the spectacles and invisible without, are visible again in far greater detail, a difference in keeping with how much better he sees physical things with the mask than he did before. He can also see a shifting glimmer in the mist, an indication that it has a spiritual aspect; he can see his own soul embodied in himself, if he looks closely enough at his body, and when the mist rises and drops again to reveal a tall standing mirror, he can see the spirit bound up in his mask, ethereal scales twining impossibly under its surface. The soulsight layer is noticeably separate from the physical layer, and has noticeably different boundaries and limitations—different things occlude it, to a different degree—but, looking at any given object or place in the world, the two forms of sight meld seamlessly into a unified whole.
On top of that is a hint of something else. He can, just barely, access a third layer of sight that shows him not sense-data but pure information, knowledge about the things he looks at. It's vague and elusive and largely overwhelmed by the other two, but it detects the Assistant's attitude of excited pride in their workmanship, and Cold River's thoughtful curiosity, and the fact that the mist is the closest thing the Assistant has to a body.
He also breathes easier, with the mask on, like his body just has more energy than it did a minute ago. Logically speaking this is probably the passive effect of the Spirit rune—but he doesn't need logic to know that; he can feel it, with the mask-sense his mask also grants him. It's a mask-maker's mask-sense, attuned for fine detail at short range instead of clear warnings at long. With it, he can perceive the exact shape and nature of his mask, and the movement of the spirit within as she examines it.
(The Assistant does good work,) comes a soundless whisper into the back of his thoughts. Cold River communicates more in concepts than in words, although it seems she's starting to pick up his languages through the bond, because the concepts are structured partly using his own vocabulary. (I think we will do very well together, you and I and this mask.)