“Here I have one of the most modern dark-field microscopes,” he resumed.  “On this slide I have placed a little pin-point of a culture made from the blood of Saratovsky.  I will stain the culture.  Now—­er—­Walter, look through the microscope under this powerful light and tell us what you see on the slide.”

I bent over.  “In the darkened field I see a number of germs like dancing points of coloured light,” I said.  “They are wriggling about with a peculiar twisting motion.”

“Like a corkscrew,” interrupted Kennedy, impatient to go on.  “They are of the species known as Spirilla.  Here is another slide, a culture from the blood of Samarova.”

“I see them there, too,” I exclaimed.

Every one was now crowding about for a glimpse, as I raised my head.

“What is this germ?” asked a hollow voice from the doorway.

We looked, startled.  There stood Saratovsky, more like a ghost than a living being.  Kennedy sprang forward and caught him as he swayed, and I moved up an armchair for him.

“It is the spirillum Obermeieri,” said Kennedy, “the germ of the relapsing fever, but of the most virulent Asiatic strain.  Obermeyer, who discovered it, caught the disease and died of it, a martyr to science.”

A shriek of consternation rang forth from Samarova.  The rest of us paled, but repressed our feelings.

“One moment,” added Kennedy hastily.  “Don’t be unnecessarily alarmed.  I have something more to say.  Be calm for a moment longer.”

He unrolled a blue-print and placed it on the table.

“This,” he continued, “is the photographic copy of a message which, I suppose, is now on its way to the Russian minister to France in Paris.  Some one in this room besides Mr. Jameson and myself has seen this letter before.  I will hold it up as I pass around and let each one see it.”

In intense silence Kennedy passed before each of us, holding up the blue-print and searchingly scanning the faces.  No one betrayed by any sign that he recognised it.  At last it came to Revalenko himself.

“The checkerboard, the checkerboard!” he cried, his eyes half starting from their sockets as he gazed at it.

“Yes,” said Kennedy in a low tone, “the checkerboard.  It took me some time to figure it out.  It is a cipher that would have baffled Poe.  In fact, there is no means of deciphering it unless you chance to know its secret.  I happened to have heard of it a long time ago abroad, yet my recollection was vague, and I had to reconstruct it with much difficulty.  It took me all night to do it.  It is a cipher, however, that is well known among the official classes of Russia.

“Fortunately I remember the crucial point, without which I should still be puzzling over it.  It is that a perfectly innocent message, on its face, may be used to carry a secret, hidden message.  The letters which compose the words, instead of being written continuously along, as we ordinarily write, have, as you will observe if you look twice, breaks, here and there.  These breaks in the letters stand for numbers.