Unless his accursed imagination had been playing him tricks, a trap of some kind had been opened above his head and someone had looked in at him; yet—­and his fingers were trained to such work—­he was prepared to swear that the surface of the Chinese paper covering the wall was perfectly continuous.  He drummed upon it lightly with his finger-tips, here and there over the surface above the bed.  And in this fashion he became enlightened.

A portion, roughly a foot in height and two feet long, yielded a slightly different note to his drumming; whereby he knew that that part of the paper was not attached to the wall.  He perceived the truth.  The trap, when closed, fitted flush with the back of the wall-paper, and this paper (although when pasted upon the walls it showed no evidence of the fact) must be transparent.

From some dark place beyond, it was possible to peer in through the rectangular patch of paper as through a window, at the occupant of the bunk below, upon whom the shaded lamp directly poured its rays!

He examined more closely a lower part of the wall, which did not fall within the shadow of the purple lamp-shade; for he was thinking of the draught which had followed the opening of the trap.  By this examination he learnt two things:  The explanation of the draught, and that of a peculiar property possessed by the mural decorations.  These (as Soames had observed before him) assumed a new form if one stared at them closely; other figures, figures human and animal, seemed to take shape and to peer out from behind the more obvious designs which were perceptible at a glance.  The longer and the closer one studied these singular walls, the more evident the under design became, until it usurped the field of vision entirely.  It was a bewildering delusion; but M. Max had solved the mystery.

There were two designs; the first, an intricate Chinese pattern, was painted or printed upon material like the finest gauze.  This was attached over a second and vividly colored pattern upon thick parchment-like paper—­as he learnt by the application of the point of his pocket-knife.

The observation trap was covered with this paper, and fitted so nicely in the opening that his fingers had failed to detect, through the superimposed gauze, the slightest irregularity there.  But, the trap opened, a perfectly clear view of the room could be obtained through the gauze, which, by reason of its texture, also admitted a current of air.

This matter settled, M. Max proceeded carefully to examine the entire room foot by foot.  Opening the door in one corner, he entered the bathroom, in which, as in the outer apartment, an electric light was burning.  No window was discoverable, and not even an opening for ventilation purposes.  The latter fact he might have deduced from the stagnation of the atmosphere.