Sorry for the many reblogs but I found more quotes:
A bright silver light appeared right ahead of him, moving through the trees.
He screwed up his eyes as the light became blinding, the trees in front of it pitch-black in silhouette, and still the thing came closer...
And then the source of the light stepped out from behind an oak. It was a silver-white doe, moon-bright and dazzling, picking her way over the ground, still silent, and leaving no hoofprints in the fine powdering of snow.
Snow crunched beneath his feet, but the doe made no noise as she passed through the trees, for she was nothing but light.
Though the darkness had swallowed her whole, her burnished image was still imprinted on his retinas; it obscured his vision, brightening when he lowered his eyelids, disorienting him. Now fear came: Her presence had meant safety.
The imprint of the doe faded away with every blink of his eyes as he stood there, listening to the sounds of the forest, to distant crackles of twigs, soft swishes of snow.
The Doe becomes so bright that it doesn’t remain silver, it becomes white. Notice how many words focus on her light and brightness. She’s blinding. Harry cannot see anything for a time, he’s disoriented, blinded as he was by her brightness. He keeps seeing her for several moment on his retinas after she’s gone, like when you’ve been in out in the sun for long and you enter back into a dark place, seeing a greenish light everywhere instead of what’s in front of you.
Snape’s Doe may be the most powerful Patronus we’ve ever seen... or one of the most powerful at the very least.