"In what way is it done?"

"Simply by using light as the agency."

"Is it like a telescope?"

[Ill.u.s.tration: _Fig. 31. The color Spectrum._]

"No; entirely different. It depends wholly on one thing, and that is the breaking up or dividing the light that comes from an object. Let me make this a little plainer. If a ray of sunlight is allowed to pa.s.s through an orifice into a darkened room, and in the transit through the opening it goes through a prism, or three-sided piece of gla.s.s, the light produced on the opposite wall will show the seven colors of which sunlight is composed. The drawing (Fig. 31) shows how this is arranged.

Now iron shows these colors differently arranged, aluminum in another way, and so on with all different substances, and the light projected from each is called its spectrum, its particular a.n.a.lysis."

John's recital during the day had produced a powerful impression on all, as well it might. It shows what wonderful trials men can endure.

Ralph and Tom were frequently affected by it, and at times could not prevent tears from coming. They recalled their own sufferings.

The Professor thanked John that evening for his story, and said: "We must not tax the patient with any more talk to-night. We have learned a lesson of perseverance and trials. The history of man is always profitable, and we are thankful for the news it gives us of the people here but you must be patient and wait a more opportune time to hear our story, and then we can advise with each other as to our future course."

The boys were early in conference with each other after they left John, because there were some interesting things to them in John's story, which needed clearing up.

"Did you hear what he said about that cave?" was Ralph's first question.

"Yes; and I think I know where that cave is?" answered Harry.

"Where?" asked Tom and George in a breath.

"Right at the camp where we found you."

"Oh, you mean that cave we found at the hillside after we started for the river?"

"Don't you recall that John took me around to the mouth of the cave, and when we said that we might use that to hide in, he shook his head, and moved away?"

"I thought the Professor acted queerly about it, too, because he urged us away from the place."

All remembered the circ.u.mstance, and they also recalled that the Professor gave a vague reply when they asked him the reason why.

George cast a scrutinizing glance at Harry, who waited for him to speak.

"Harry, do you think he found any treasure in that cave?"

Ralph and Tom now opened their eyes in wonder. Was that what he meant when he said there was something wonderful there? Harry looked at the boys for a moment, in the intensity of the situation, and said: "And we have also found a cave." But the eyes of George caught Harry, who suddenly stopped, because he recalled their agreement not to divulge it to the boys until the matter was mentioned to him.

"Where is it?" exclaimed Tom, eagerly.

"Not far from here."

"Will it be much trouble to visit it?"

"No; and we shall probably do so some day."

An island full of bitter and vindictive savages, and a handful of men to meet them. It looked, indeed, like a hopeless task. John's story left many things unsaid; many things that they longed to know. Who were Wright and Walters, whose names were in the note found in the _Investigator's_ lifeboat, and who was Will, the writer of the note?

The Professor was just as anxious as the boys to have those matters cleared up, but he knew it would be unwise to tax his strength with a further recital, and the inevitable questions which would be propounded, and it was well that his injunctions were followed, because he was not yet well by any means, and the further news which they awaited was postponed.

In the evening Ralph had the flute, but the ba.s.s viol was not yet ready, so that the two instruments gave a little diversion to the day of excitement and wonder.

John's illness did not now interfere with the work on the house. It was pushed forward with the greatest energy, the roof and sides enclosed, and they were now nearly ready for occupying it, by the time John was again able to be about.

Shortly after they had made the first samples of gla.s.s, some months before, the trip to the west had postponed the work in that direction, and the Professor, with the aid of George, turned out the first samples of gla.s.s, which they intended to use in the new building.

"Why can't we have a looking-gla.s.s? It would be such fun to set up several of them."

"I think we have sufficient mercury for the purpose," answered the Professor; so calling in Ralph they set to work, under the Professor's direction, to make some mirrors.

"The princ.i.p.al thing in mirrors is to get a white reflecting surface.

Silver and mercury are metals which lend themselves to that use. If you polish anything bright enough it will serve as a mirror, but the whiter the surface is the better."

"Then why wouldn't white paper be the best?"

"It would if you could get a fine polish on its surface, but the finest surface on the densest paper is not as smooth as the polished surface of the metals."

"What is the best way to make the mirror?"

"The most available plan for us to follow is to make an amalgam of tin and mercury."

"But what do you mean by an amalgam?"

"It means the combination of mercury, or quicksilver, with any other metal."

"Will it be difficult to combine tin and mercury, so as to make an amalgam?"

"That is one of the simplest things in the arts. Tin and mercury unite by merely rubbing them together; see how easily they combine to form just such a surface as you want."

"Isn't that fine? But as that shines so nicely, what is the need of putting a gla.s.s over it?"

"Simply to protect the amalgamated surface."

The largest piece of gla.s.s thus far made was sixteen by twenty-four inches, and the boys selected the most perfect pane, and in a short time a very good mirror had been turned out.

"It has occurred to me that it would be good policy to make a number of small mirrors, say six inches square. They would be a valuable a.s.set to us in our next expedition."

This opened the eyes of the boys to the commercial utility of the work they were engaged in for the first time. George rushed over and brought Tom and Harry to the laboratory, and exhibited the mirrors, and explained that they intended to make a number of small ones to take with them.

"That is a capital idea. Won't the natives go wild over them?"

They were at work at once, first cutting up some of the gla.s.s the requisite size, and before the afternoon closed they had several small ones in addition to the large one.

The large one was carried over to the living room, and when it was brought in and hung against the wall John's face lighted up, when they told him of the work required to turn out the gla.s.s, and to make the amalgam.

"What a glorious opportunity you boys are having. How anxious I am to get up and help you. What a splendid mirror that is. You surprise me with the character of your work."

"We are going to have real windows in the new house."