Reading the Harry Potter series the first time around was a deeply stressful experience. Before the movies came out, before Twitter and Tumblr and heartbreaking Harry Potter quote tattoos, we actually didn't know how this story was going to end. So when, in the opening scene of Harry Potter and the Half-Blood Prince , we found Snape making an Unbreakable Vow to help Draco in a mission from the Dark Lord, we felt like we had been totally betrayed. We didn't know back then that he was working on Dumbledore's orders; we weren't yet crying every time we heard the world "always"; we didn't know that all would be well in the end. But that's because there was one massive clue that we missed.
To reassure ourselves, all we needed to do was flick back a few books to Goblet of Fire . Because there, the truth was staring us in the face: there was no way Snape could have been a bad guy, and the Foe-Glass can prove it.
The Foe-Glass, as Potterheads will recall, is a magical mirror that belonged to the fake Mad-Eye Moody. He explained to Harry that it revealed his enemies inside it — and he knew he was in danger once he could see the whites of their eyes. While it's never said explicitly, we can assume that the mirror shows the enemies of its possessor rather than the enemies of whoever looks inside it, because Harry sees the same shadowy figures that the fake Moody is pointing to, rather than — oh, I don't know — his own dangerous enemy sitting right in front of him.
At the end of the book, fake Moody is revealed to be Barty Crouch Jr. in disguise. Barty is a Death Eater, and most definitely a bad guy — so it goes without saying that any enemy of Barty Crouch Jr. has got their moral compass pointing in the right direction.
Well, cast your mind back to the scene where Dumbledore, McGonagall and Snape burst into Moody/Crouch's office to rescue Harry. All three of them appear in the Foe-Glass, meaning all three of them are enemies of Crouch, and by extension, enemies of the Death Eaters. Yep, it's right there on the page. We needn't have gone through all that worry and heartache; J.K. Rowling had already made it perfectly clear that Snape was not secretly a Death Eater.
And there's an even more subtle clue: when Snape arrives, he pointedly looks at his own reflection in the Foe-Glass. It's a pretty high-drama situation, so why would he waste time looking in the mirror? Well, just a few hours before, thanks to the Dark Mark on his arm, Snape found out that Voldemort was back — and that Snape was going to be summoned back to join him. Of course, what Voldemort didn't know was that Snape had entirely rejected his old Death Eater beliefs, and would now be unwaveringly loyal to Dumbledore.
If Snape wanted to work as a spy, and fool Voldemort, he would no doubt be pretty anxious about being discovered. So it makes sense that he would want to check and see if magical instruments like the Foe-Glass would blow his cover — which explains why he would look in the mirror at the first chance he got.
This detail only makes what Snape did even more brave: the Foe-Glass showed him that he was at risk of being exposed if Voldemort ever magically tested his loyalty (and Voldemort's wrath is not something you want to be on the receiving end of) — and yet Snape still did what Dumbledore asked. Oh J.K., it's been nine years and you still can't stop making me cry, can you?
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