“That's easy, Harry. I can give you my theory in a few short words. You've studied physiology, haven't you? Well, that's where you can get your proof--or rather let me say my theory. What is the Blind Spot?” “In optics?” “We'll forgo that,” he answered. “I refer to this one.” I thought for a moment. “Well,” I said, “I don't know. It was something I couldn't see. Watson went out before our eyes. He was lost.” “Exactly. Do you get the point?” “No.” “It is this. What you see is merely energy. Your eye is merely a machine. It catches certain colours. Which in turn are merely rates of vibration. There is nothing to matter but force, Harry; if we could get down deep enough and know a few laws, we could transmute it.” “What has it to do with the occult?” “Merely a fact. The eye machine catches only certain vibration speeds of energy. There are undoubtedly any number of speeds; the eye cannot see them.” “Then this would account for the Blind Spot?” “Exactly. A localised spot, a condition, a combination of phenomena, anything entering it becomes invisible.” “Where does it go to?” “That's it. Where? It's one of the things that man has been guessing at down the ages. The professor is the first philosopher with sound sense. He went after it. It's a pity he was trapped.” “By the Rhamda?” “Undoubtedly.” “Who is he?” Hobart smiled. “How do I know? Where did he come from? If we knew that, we would know everything. 'A phantom,' so Watson says. If so, it only strengthens our theory. It would make a man and matter only a part of creation. Certainly it would clear up a lot of doubts.” “And the ring?” “It controls the Blind Spot.” “In what way?” “That's for us to find out.” “And Watson? He is in this land of doubt?” “At least he is in the Blind Spot. Let me try the ring.” He struck a match. It was much as it had been in the restaurant, only a bit more startling. Then the blue faded, the colour went out, and it became transparent. For a moment. There was an effect of space and distance that I had not noted before, almost marvellous. If I could describe it at all, I would say a crystal corridor of a vastness that can scarcely be imagined. It made one dizzy, even in that bit of jewel: one lost proportion, it was height, distance, space immeasurable. For an instant. Then the whole thing blurred and clouded. Something passed across the face; the transparency turned to opaqueness, and then--two men. It was as sudden as a flash--the materialisation. There was no question. They were alive. Watson was with the professor. It was a strange moment. Only an hour before one of them had been with us. It was Watson, beyond a doubt. He was alive; one could almost believe him in the jewel. We had heard his story: “The screen of the occult; the curtain of shadow.” We had seen him go. There was an element of horror in the thing, and of fascination. The great professor! The faithful Watson! Where had they gone?