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“We knew they were on the coast,” said the admiral, a little later, when Metcalf had made his report on the quarter-deck of the Delaware. “But about this light? Are you sure of all this? Why, if it’s so, the President will rank you over us all. Mr. Smith came in with the prisoners, but he said nothing of an invisible light–only of a strong searchlight with which you set fire to the signal-yard.”

“I did not tell him all, admiral,” answered Metcalf, a little hurt at the persistence of the feeling. “But I’m satisfied now. That fleet is coming on with incompetents on the bridge.”

“Well, we’ll soon know. I’ve only one ship, but it’s my business to get out and defend the United States against invaders, and as soon as I can steam against this gale and sea I’ll go. And I’ll want you, too. I’m short-handed.”

“Thank you, sir. I shall be glad to be with you. But wouldn’t you like to examine the light?”

“Most certainly,” said the admiral; and, accompanied by his staff, he followed Metcalf aboard the submersible.

“It is very simple,” explained Metcalf, showing a rough diagram he had sketched. “You see he has used my system of reflectors about as I designed it. The focus of one curve coincides with the focus of the next, and the result is a thin beam containing nearly all the radiations of the arc.”

“Very simple,” remarked the admiral, dryly. “Very simple indeed. But, admitting this strong beam of light that, as you say, could set fire to that sealer, and be invisible in sunshine, how about the beam that is invisible by night? That is what I am wondering about.”

“Here, sir,” removing the thick disk from around the light. “This contains the prisms, which refract the beam entirely around the lamp; and disperse it into the seven colors of the spectrum. All the visible light is cut out, leaving only the ultraviolet rays, and these travel as fast and as far, and return by reflection, as though accompanied by the visible rays.”

“But how can you see it?” asked an officer. “How is the ship it is directed at made visible?”

“By fluorescence,” answered Metcalf. “The observer is the periscope itself. Any of the various fluorescing substances placed in the focus of the object-glass, or at the optical image in front of the eyepiece, will show the picture in the color peculiar to the fluorescing material. The color does not matter.”

“More simple still,” laughed the admiral. “But how about the colored lights they saw?”

“Simply the discarded light of the spectrum. By removing this cover on the disk, the different colored rays shoot up. That was to attract attention. I used only white light through the periscope.”

“And it was this invisible light that blinded so many men, which in your hands blinded the crews of the Japanese?” asked the admiral.

“Yes, sir. The ultraviolet rays are beneficial as a germicide, but are deadly if too strong.”

“Lieutenant Metcalf,” said the admiral, seriously, “your future in the service is secure. I apologize for laughing at you; but now that it’s over and you’ve won, tell us about the spectacles.”

“Why, admiral,” responded Metcalf, “that was the simplest proposition of all. The whole apparatus–prisms, periscope, lenses, and the fluorescing screen–are made of rock crystal, which is permeable to the ultraviolet light. But common glass, of which spectacles are made, is opaque to it. That is why near-sighted men escaped the blindness.”

“Then, unless the Japs are near-sighted, I expect an easy time when I go out.”

But the admiral did not need to go out and fight. Those nine big battle-ships that Japan had struggled for years to obtain, and the auxiliary fleet of supply and repair ships to keep them in life and health away from home, caught on a lee shore in a hurricane against which the mighty Delaware could not steam to sea, piled up one by one on the sands below Fort Point; and, each with a white flag replacing the reversed ensign, surrendered to the transport or collier sent out to take off the survivors.