As for the group remaining with Watson, not one of them ever dreamed that any danger might come out of the Blind Spot.  Its manifestations had been local and mostly negative.  No; the main incentive of their interest had been simply curiosity.

But apparently Watson was above them all.  He paid no further attention to them for a while; he bent at Fenton’s desk and worked swiftly.  At length he thrust his papers aside.

“I want to see that cellar,” he announced.  “That is, the point where you found that red pebble!”

Down in the basement, Sir Henry gave the details.  When he came to mention the various liquids which Fenton had poured into the woodwork upstairs Watson examined the pool intently.

“Quite so.  They would come out here—­naturally.”

“Naturally!”

Sir Henry could not understand.  His perplexity was reflected in the faces of Herold, the two physicians, Dr. Malloy, Miss Clarke, and Mme. Le Fabre—­and Charlotte spoke for them all: 

“Can’t you explain, Mr. Watson?  The woodwork had nothing whatever to do with the cellar.  There was the floor between, just as you see it now.”

“Naturally,” Watson repeated.  “It could be no other place!  It was on its way to the other side, but it could go only half-way.  Simply a matter of focus, you know.  I beg pardon; you must hold your curiosity a little longer.”

He began measuring.  First he located the line across the floorjoists overhead, where rested the partition separating the dining-room from the parlour.  Finding the middle of this line, he dropped an improvised plumb-line to the ground; and from this spot as centre, using a string about ten feet long, he described a circle on the earth.  Then, referring to his calculations, he proceeded to locate several points with small stakes pressed into the soil.  Then he checked them off and nodded.

“It’s even better than the professor thought.  His theory is all but proven.  If Jerome and Hume can deliver the other stone without accident, we can save those now inside the Spot.”  Then, very solemnly:  “But we face a heavy task.  It will be another Thermopylae.  We must hold the gate against an occult Xerxes, together with all his horde.”

“The hosts of the dead!” exclaimed Mme. Le Fabre.

“No; the living!  Just give me time, Madame, and you will see something hitherto undreamed of.  As for your theory—­tomorrow you may doubt whether you are living or dead!  In other words, Dr. Holcomb has certainly proved the occult by material means.  He has done it with a vengeance.  In so doing he has left us in doubt as to ourselves; and unless he discovers the missing factor within the next few hours we are going to be in the anomalous position of knowing plenty about the next world, but nothing about ourselves.”

He paused.  He must have known that their curiosity could not hold out much longer.  He said: