"In what way is it done?"
"Simply by using light as the agency."
"Is it like a telescope?"
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 pass through an orifice into a darkened room, and in the transit through the opening it goes through a prism, or three-sided piece of glass, 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 analysis."
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.