Inside a minute a light sprang up from the contact.  Immediately Harry and Chick beheld something they had not seen on the wall—­a knob, or button.  The doctor pulled sharply on it.  Instantly a door opened in the wall.

They passed into another room.  It was not a large place—­about thirty feet across, perhaps, stone-walled and with a low ceiling.  From all sides a soft, intrinsic glow was given off.  There were no furnishings.

But in the centre of the ceiling, occupying almost all the space overhead, a snow-white substance hung as if suspended.  Were it not for its colour and its size, it might have been likened to an immense, horizontal grindstone hung in mid-air, with apparently nothing to hold it there.  Around its side they could make out a narrow gap between it and the ceiling.  And directly along its lower edge was a series of small, fiery jewels inset, and of the order and colour of the sign of the Jarados—­red, blue and green, alternating.

The professor produced an electric torch and held it up to show that the gap between the stone and the ceiling was unbroken at any point.  Then he counted the jewels on the lower edge.  Chick made out twenty-four.  Three were missing from their sockets—­all told, then, there should have been twenty-seven.

The doctor noted the positions of the three empty sockets and, drawing a tapeline from his pocket, proceeded to measure the distances from each of the three—­they were widely separated round the circle—­from each other.  Then he turned to Chick and Harry.

“Do you know where we are?”

“Under the Spot of Life,” it was easy to answer.

“You are in San Francisco!”

“Not in—­in—­” Chick hesitated.

“Yes.  Exactly.  This is 288 Chatterton Place—­the house of the Blind Spot.”  He paused for them to digest this.  Then, “Harry—­did you say Hobart Fenton was with you on that last night?”

“Hobart and his sister, Charlotte.  I remember their coming at the last minute.  They were too late, sir.”

The professor nodded.

“Well, Harry, the chances are that Hobart is not more than twenty feet away at the present moment.  Charlotte may be sitting right there”—­pointing to a spot at Harry’s side—­“this very instant.  And there may be many others.

“No doubt they are working hard to solve the mystery.  Unfortunately the best they can do is to guess.  We hold the key.  That is—­I should correct that statement—­we hold the knowledge, and they hold the keys.”

“The keys?” Harry wanted to know more.

The professor pointed to the three empty sockets in the great white stone above their heads.  “These three missing stones are the keys.  Until they are reset we cannot control the Spot.  I had found two of them before I came through.  I take it that both of you remember the blue one?”

“I think,” agreed Chick, “that neither of us is ever likely to forget it!  Eh, Harry?”