“The three of us,” said he, “are going to take a look at the under side of the Blind Spot. This stairway leads into a secret chamber inside the foundations of the great stair; and according to this data I found in the palace, together with some calculations of my own, we ought to find some of the secrets of the Spot.” He led the way up the steps. At the top of the flight they came to a blank, blue wall. There was no sign of a door, but in the front of the wall stood a low platform, in the centre of which was set a strange, red stone. The professor consulted his chart, then opened his black case. From it he took another stone, red like the other, but not so intense. This he touched to the first, and waited. 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.