sky.

"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 sa