Why does Marsh burn tin in the dark?

Question

This question contains spoilers for the first part of the Mistborn trilogy (and maybe minor spoilers for the second part, not sure). You've been warned.

At the end of chapter 69 of The Hero of Ages we have this bit:

Marsh held up the sheet, flaring his tin to get a better look at its contents in the darkness.

Why would tin help Marsh see in the darkness, and why would he even have trouble seeing in the dark? The way I understood previous descriptions of how an Inquisitor's vision works, they're not dependent on light at all (I mean, they don't even have eyes...). By my understanding all they "see" are the blue Allomantic lines caused by burning iron and steel, but because their command of those metals is a lot more subtle than a normal Allomancer's they can see effectively like normal people (or better).

I also vaguely remember Marsh walking into a dark room before, probably at the Conventical of Seran.

Is this a rare slip by Sanderson, or is there a reason why an Inquisitor would need to enhance its vision in the dark by burning tin? Or did I just misunderstand how their vision works?

(I've finished reading the trilogy, but haven't read any other Cosmere works yet. If the answer to this spoils any major plot points in other works, please add a warning or use spoiler tags.)

Answer 1

Tin is the Internal Physical Pulling metal. It enhances sensitivity to all five senses, and cause a huge burst of sensory input when flared (coppermind).

Brandon has described several of the allomantic abilities that "show" the Allomancer things as being able to be sensed by the blind. (Gold, and Atium are the metals sited here) (Emphasis mine)

A blind person would indeed sense these things, but not have the vision with the eyes. In the same way that a blind person still dreams, but doesn't "See" in them. (As I understand it.) I'd suggest talking to someone who is blind and getting their take on how this would work.

This would imply that the blue lines that the Inquisitors use to "see" are merely perceived as sight for most Allomancers because they have eyes.

I would conjecture that because of the "huge burst of sensory input" described in flaring Tin, combined with the less concrete concept of what sensory information is in the Mistborn universe, that flaring Tin would in fact let someone who relies on allomancy for sight, "see" better.

Sanderson's writing here could be setting the scene (it is dark), and describing the fact that Marsh is having to strain to see the writing (because of the way that the Inquisitors see I would imagine that it is difficult to make out scratching in a metal sheet). In all reality though it is probably just an oversight by Brandon that we are now having to scramble to make up an in-universe answer for. :)

Answer 2

From Chapter 38 of The Final Empire:

He couldn’t see anymore, not as he once had, but he had been given something better. A command of Allomancy so subtle, so detailed, that he could make out the world around him with startling accuracy.

Almost everything had metal in it—water, stone, glass . . . even human bodies. These metals were too diffuse to be affected by Allomancy—indeed, most Allomancers couldn’t even sense them.

With his Inquisitor’s eyes, however, Kar could see the iron-lines of these things—the blue threads were fine, nearly invisible, but they outlined the world for him. The obligators before him were a shuffling mass of blues, their emotions—discomfort, anger, and fear—showing in their postures.

This is how Inquisitors see; Sazed does at one point point out that they don't need light to see. So the tin helps him see the light brighter and in more detail, enabling him to read the thing.

However, this doesn't explain why the book mentions "in the darkness." I can only assume this was an error on Sanderson's part. It's also possible that, since Inquisitors are a Hemalurgic creation and therefore of Ruin, who can't change metal, it's harder for them to read lines in metal.