"He saw a white shirt cuff, a bit of the sleeve of a coat.
You're not going to get over that, you know.

He's certain of it!" "Varner may be as certain of it as he likes," answered Archdale, almost indifferently, "and still he may be mistaken.

The probability is that Varner was confused by what he saw.

He may have had a white shirt cuff and the sleeve of a black coat impressed upon him, as in a flash--and they were probably those of the man who was killed.

If, as I suggest, the man slipped, and was shot out of that open doorway, he would execute some violent and curious movements in the effort to save himself in which his arms would play an important part.