"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."