The fog had grown thicker; one could almost taste it.  I couldn’t suppress a shudder.  It was cold, dank, repressive.  Neither of us spoke a word on our way downtown.  Hobart opened the door to our apartment; he turned on the lights.

In a few moments we had hot, steaming cups of coffee.  Still we did not speak.  Hobart sat in his chair, his elbows on the table and his head between his hands.  My thoughts ran back to that day in college when he said “I was just thinking, Harry, if I had one hundred thousand dollars, I would solve the Blind Spot.”

That was long ago.  We had neither of us thought that we would come to the fact.

“Well,” I spoke, “have you got that hundred thousand dollars?  You had an idea once.”

He looked up.  “I’ve got it yet.  I am not certain.  It is merely a theory.  But it’s not impossible.”

“Well, what is it?”

He took another drink of coffee and settled back in his chair.

“It is energy, Harry—­force.  Nothing but energy—­and Nature.”

“Then it’s not occult?” I asked.

“Certainly it is.  I didn’t say that.  It is what the professor promised.  Something concrete for our senses.  If the occult is, it can certainly be proven.  The professor was right.  It is energy, force, vibration.  It has a law.  The old doctor was caught somehow.  We must watch our step and see that we aren’t swallowed up also.  Perhaps we shall go the way of Watson.”

I shuddered.

“I hope not.  But explain.  You speak in volumes.  Come back to earth.”

“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.”