upon returning to his laboratory, what does the TT see Mrs. Watchett do?

From the text:

Mrs. Watchett had walked across the room, travelling, as it seemed to me, like a rocket. As I returned, I passed again across that minute when she traversed the laboratory. But now her every motion appeared to be the exact inversion of her previous ones. The door at the lower end opened, and she glided quietly up the laboratory, back foremost, and disappeared behind the door by which she had previously entered. Just before that I seemed to see Hillyer for a moment; but he passed like a flash.