“Good.  We may need them, although I hope not.  Fortunately the Spot is small, and a few of us can hold it against a good many.  What we must do is to extricate our friends and close it.  Afterward we may have time for more leisurely investigation.  But we must remember, above all things, that black case of Professor Holcomb’s!  It holds the secrets.

“Now I must ask you all to step out of this room.  This library, you know, is the Blind Spot.”

He directed them to take positions along the balustrade of the stairway, out in the hall—­through the wide archway, where they could have a clear view, yet be safe.

It was a curious test.  With nothing but his mathematics and his drawing to go by, Watson was about to set the three stones in their invisible sockets.  He spread the map out carefully, likewise his calculations; they gave him, on this floor, the precise positions that he charted on the earth of the cellar.  A glance toward the front of the house—­north—­then a little measuring, three chalk-marks on the carpet, and he was ready for the final move.

He took the fateful ring and with a penknife pried up the prongs that held the stone.  As it popped out he caught it with one hand.  Then he looked at the row of wondering faces along the stair.

“I think it will work,” he said.  “But, remember—­don’t come near!  I shall get out as best I can myself; don’t try to save me.”

With that he held the jewel on the first of the three chalk-marks on the circumference of the great circle.  He held it tight against the carpet and then let go.  Up it flashed about one foot—­and disappeared.

There was no sound.  Next Watson took the red stone.  With it, the process was inverted.  Instead of holding it to the floor he raised it as high as he could reach, directly above the second mark.  Then he let it drop.

It did not reach the floor.  It fell a little more than halfway, and vanished.

The third stone, the green one, was still remaining.  Watson took it to the third and final mark on the circle, taking care to keep outside the circumference that marked the Spot.  This mark was directly in front of the archway.  He turned to them.

“Watch carefully,” he spoke.  “I do not know what has transpired in the temple during the past few hours.  Be ready for anything.  All of you!”

He dropped the stone.

With the same motion he dodged out into the hall.

Though there was no sound there was something that every one felt—­a sibilant undertone and cold vibration—­a tense flash of magnetism.  Then the dot of blue—­a string of incandescence; just as had been spoken.

The Blind Spot was opening.

Watson silently warned the others to remain where they were and himself crowded back against the stair.  And as he did so, someone came noiselessly down the steps from the floor above, passed unnoticed behind the watchers and thence across into the hall.