Stanza 1 Summary

Get out the microscope, because we’re going through this poem line-by-line.

I am silver and exact. I have no preconceptions.

While "Sylvia" might sound a little bit like silver, we're pretty sure from this first-person declaration that Plath isn't the speaker in this poem. Instead, we have to think back to the title: what is silver and exact? Well, a mirror!
We know mirrors don't talk – but that just makes us more curious about what this mirror is going to say.
We know from looking at them that mirrors are silver and give an exact reflection of what is in front of them.
The second part of the line is not so simple. This mirror is telling us it has no preconceptions. The mirror doesn't change what it shows you based on it's understanding of who you are, or whether you're having a bad day or a good day – it just shows what it sees. So, while this mirror may be personified in the poem, it doesn't, like most people, let what it has seen before affect what it does in the present.

Whatever I see, I swallow immediately.

Now the personification becomes a little weirder. We can imagine a person who is exact, who has no preconceptions, but a person who swallows everything he sees – now that's a stretch.
To figure out this line, it helps to think of what mirrors do to everything they see – they reflect it. Swallowing everything, then, is a metaphor for reflecting everything.
The substitution of "swallowing" for "reflecting" makes this mirror seem human. It appears hungry to us, and a little unforgiving and scary. We certainly don't want to be swallowed by our mirrors.
In terms of sound, the rhythm of this line swallows the reader right up; it's arranged to be sharp and deliberate, but reads like a riddle.

Just as it is, unmisted by love or dislike

This line is giving us more information about how the mirror is swallowing what it sees, while also confirming what we already know from the first line: that the mirror is exact and has no preconceptions.
The first part of this line isn't too complicated – we know that mirrors reflect things just as they are.
But then we get to the second part of the line, where we find out that whatever the mirror swallows is "unmisted by love or dislike." Unmisted is yet another metaphor; here it means unchanged, but it gives us an image of an actual mist that could be – but isn't – clouding what the mirror sees.
Even more interesting, love and dislike are the things that cause this mist. The mirror, even though it's not human, knows that when humans love something, it appears more beautiful, and when we dislike something, it seems uglier.
But the mirror is beyond all this. The vision-impairing mist of love or dislike does not apply to the mirror, which shows things exactly as they are.

I am not cruel, only truthful –
The eye of a little god, four-cornered.

Here the mirror seems to realize that it's coming off as a little harsh, because it just shows what it sees and takes nothing else in account. So it explains that it's not cruel, just truthful. If the mirror were to lie to make what it reflected look worse than it already does, it would be considered cruel. Instead, it just shows what it sees, good or bad.
We now get a dash connecting line 4 to line 5. A dash can mean many things (check out the poems of Emily Dickinson), but here, it seems to denote a comment from the mirror, explaining the previous line further, while in the meantime giving us a pretty cool new way to think about a mirror.
The mirror, in line 5, is comparing itself to the eye of a "little god." Indeed, the mirror is getting a little high and mighty here, saying that it's powerful. It's also saying something about what it thinks a god is like – not cruel, but truthful.
Notice that the word "god" isn't capitalized in this line: it could refer to any god, even one in the guise of a mirror.
Finally, the note that the god's eye is "four-cornered" (square or rectangular) helps us complete – in a concise and graceful way – the image of the eye in the shape of a mirror.

Most of the time I meditate on the opposite wall.

This line tells us in a roundabout way what the mirror is facing: a wall.
The line continues to personify the mirror – instead of facing it, or reflecting it, the mirror "meditates on" (or contemplates) the opposite wall. This implies that the mirror, an inanimate object, thinks.

It is pink, with speckles. I have looked at it so long
I think it is a part of my heart. But it flickers.

Now we find out more details about the opposite wall, which serves as the object of the mirror's meditation, or thoughts.
The wall is speckled and pink. The color pink makes the wall seem feminine; this mirror is probably in a girl's bedroom or bathroom.
Next, the mirror tells us about its connection to the wall. Using enjambment, a literary device where a thought is split between two lines, the mirror tells us that it has looked at this wall for so long that it feels like the wall is a part of its heart.
It's a little cute that the mirror feels like what it's reflecting is a part of its heart. But then we remember that the mirror doesn't have this feeling for the person it often reflects, but rather for a boring pink wall.
At the end of the eighth line, we see that the relationship between the wall and the mirror isn't as constant as we thought: the wall flickers.

Faces and darkness separate us over and over.

Here we see why the wall flickers – because of faces and darkness. The faces come to look in the mirror, and when they leave, they turn the light off, leaving the mirror to reflect nothing but the darkness.
The way Plath has structured this line makes us think that the mirror must be sad at this separation. If we didn't know any better, we'd think that these two lines were part of a love poem from person to her beloved, and not from a mirror to a wall.