On going to Frederic Larsan's room, I had found Larsan sound asleep.
"2nd.

The ladder.
"3rd.

I had placed Frederic Larsan at the end of the 'off-turning' gallery and had told him that I would rush into Mademoiselle Stangerson's room to try to capture the murderer.

Then I returned to Mademoiselle Stangerson's chamber where I had seen the unknown.
"The first evidence did not disturb me much.

It is likely that, when I descended from my ladder, after having seen the unknown in Mademoiselle Stangerson's chamber, Larsan had already finished what he was doing there.