Now since it had been demonstrated to me that the fifth could not have escaped, it was evident that one of the four present in the gallery must be a double--he must be himself and the murderer also.

Why had I not seen this before?
Simply because the phenomenon of the double personality had not occurred before in this inquiry.
"Now who of the four persons in the gallery was both that person and the assassin?
I went over in my mind what I had seen.

I had seen at one and the same time, Monsieur Stangerson and the murderer, Daddy Jacques and the murderer, myself and the murderer; so that the murderer, then, could not be either Monsieur Stangerson, Daddy Jacques, or myself.

Had I seen Frederic Larsan and the murderer at the same time ?--No!--Two seconds had passed, during which I lost sight of the murderer; for, as I have noted in my papers, he arrived two seconds before Monsieur Stangerson, Daddy Jacques, and myself at the meeting-point of the two galleries.

That would have given Larsan time to go through the 'off-turning' gallery, snatch off his false beard, return, and hurry with us as if, like us, in pursuit of the murderer.