"Great! But I'm afraid that I can't quite see."

"Patience, Signor Solo! Patience! To make this hologram, we set up the object to be photographed and shine our laser beam at it... but not directly! We shine it through some semi-translucent medium that both reflects and refracts... a piece of mirror that is only half silvered, for example; a fragment of frosted glass; even a sheet of plastic."

"Plastic!"

"Just so." Rinaldi was well into his stride now; Solo reckoned he was getting free half the third year optics course from Verona. "Now what happens? Half of the light penetrates the medium and goes on to illuminate the object. And the rest is reflected back towards the light source, where we allow it to fall on the photographic plate. Now the light beam which goes on through to illuminate the object also makes its way back eventually to the photographic plate. And it is the recording of the interference between these two halves of the original beam that makes the Hologram."

"What's the effect, then? On the photographic plate, I mean."

"When it is developed it makes what seems to be a meaningless blur. But wait!" Rinaldi raised a cautionary finger. "We shine the original laser beam at this blur... and, presto! we see the original object in three dimensions again!"

"I think I should understand more easily if I could see—"

"I show you! I show you!" The Colonel bounded over to his desk and opened a wide, shallow drawer. He took out what looked like an ordinary photographic plate measuring around six inches by four inches and handed it to the agent. Solo glanced at it. The ground-glass surface looked like a close-up of a piece of granite or a pattern formed by a non-regular kaleidoscope: a completely random assembly of differently colored flecks.

"That is the hologram," Rinaldi said, bustling about between the bench and a series of work tables ranged along one wall. "Now we bring it back to life for you!"

He dragged over a heavy, box-like structure bearing two dials and a row of switches, plugged a coaxial cable into the mains supply, and then attached a lead which projected from between the ventilation louvres on the steel side of the box to a complicated framework screwed to the bench. In the framework he fixed a hooded metal cylinder about twenty inches long and five inches in diameter, from one end of which projected what looked like a camera lens. "This is a ruby laser," he explained. "Only a small one—thirty joules—since it is for demonstration. But you can see what it will do..."

He arranged a somewhat shabby-looking mirror three feet away from the output aperture of the laser, took back the hologram and slotted it into a groove prepared in the bench, and walked across to draw black curtains over the opaque, reeded glass of the windows. Then he handed Solo a pair of dark glasses, flicked over a master switch beside the laser, and turned out the lights.

After a moment there was a brilliant blue-green flash from the bench followed by a deep, low humming noise. Almost at once Solo discerned a curious rose-colored fluorescence surrounding the laser cylinder; and then a painfully intense pencil of vivid crimson light pulsed from the aperture and lanced down the length of the bench. He looked further down the polished surface and gasped.

Where there had been an empty space of wood, there was now a silver tray of coffee and liqueurs laid out. Solo could see the shine on the bone china, the dark luster of coffee, the jewelled highlight lurking in the drink. He reached out his hand ... and touched nothing.

Behind him in the dark, Rinaldi chuckled. "That is what you see in our meaningless blur when the right kind of light illuminates it!" he said.

"But that's uncanny! exclaimed.

"It is impressive, no? But wait... I show you more..."

Rinaldi switched on the lights, fetched a large gilded picture frame from a corner and suspended it from the ceiling hook a few feet in front of Solo. The area circumscribed by the frame resembled the smaller hologram the agent had examined before: a wilderness of colored fragments. But as soon as the Italian reset some switches and altered the angle of the laser, the gilt framed a perfect three-dimensional color photo—a street scene in a small town, with gaily striped awnings over market stalls and a couple of cars parked by the entry to a side road.

"Remarkable! The 3-D effect is astonishing," Solo said.

"It is, yes. But perhaps more so than you realize, Signor Solo. Walk out to the side there... so... and look down that side street."

The agent whistled in amazement. As he moved out wide of the picture, craning to see "down" the narrow thoroughfare in the photo, he found that in reality he was doing just that. The back of a third car, which had not been visible before, had come into sight down the side street.

Rinaldi laughed. "Yes, you really do 'see around corners' with coherent light," he said. "It's because the hologram, as it were, freezes all the existing light waves in the original scene. They are all there, waiting to be released—or melted, if you like—when the right kind of light illuminates them. And when it does, all the original beams, not just some as in ordinary systems of stereoscopy, come back. If a thing was there to see in the original, you will be able to find it in a hologram!"

"It's fantastic. Unbelievable."

"If I had time, I could show you more. We have a holographic television demonstration. There is an actress standing in front of her dressing table, so as to hide something on the shelf behind her. You can step to one side and peer around her to see what it is!.... There is a huge Hologram transparency which works in ordinary light. If you hang it in front of a window, you see a life-size portrait of a man, apparently standing solidly in space!"

"Most interesting," Solo said. "But how exactly does it...?"

"How does it affect the problems of your Mr. Waverly? Simply enough. If an operative made a hologram of a blueprint, for example, and sent it to his headquarters, it could only be deciphered if the decoders knew how it had been made. Suppose he had made it through a sheet of frosted glass. It can only be turned back into a blueprint by shining the same light at it through the same piece of glass, held in exactly the same position."

"You mean that it would remain indecipherable unless you had that exact piece of glass and knew how it had been held...?"

"Precisely. Your spy could feel quite safe if he had sent the Hologram by one route and the glass, with the instructions how to use it, by another. Even if the hologram fell into enemy hands it would be useless without the glass."

"Suppose the glass was lost—or broken, for that matter?"

"Then it would be impossible to recover the image of the original blueprint. Without the right piece of glass it would remain as a meaningless blur!"

"That's our problem, then," Solo said. "As I understand it, Waverly has received a hologram—but he doesn't know what the man used to make it. And now the man has been killed and I have to try and find out what it was. If only Leonardo had written or cabled at the same time."

"Leonardo? Are you talking of Signor Leonardo? Is this the man who has sent the hologram to Mr. Waverly?"

"Yes, that's right. Colonel. Leonardo."

"But... but he was here!" Rinaldi was astonished. "A week ago. He has come to see me and ask if he can borrow the ruby laser for a half hour!"