“It is like air. But you cannot breathe it. It exists everywhere—even in the outermost realms of Space. Now do you understand?” “Yes,” said Tubby. “Light is vibration of the Ether,” the professor went on in his droning voice. “Its vibrations travel through the Ether at the rate of one hundred and eighty-six thousand three hundred and twenty-four miles a second.” “I know _that_,” said Tubby. “You are a learned man,” the professor said admiringly. “You have a wonderful mind. I knew you had—they said so.” “Yes,” said Tubby. “Go on.” The professor jerked his little body upright in his chair suddenly, so suddenly that Tubby started violently with fright. “How long is a second?” the professor asked aggressively. “Why—tick-tick. That’s it,” said Tubby. The professor relaxed in his chair. “You _are_ a clever man. You will have your little joke. “But we must be serious. I will tell you how long a second is. It is exactly one-thirty-one millionth, five hundred and fifty-eight thousandth, one hundred and forty-ninth part of a year.” “Oh,” said Tubby. “Do you know what a year is?” “Three hundred and sixty-five days,” said Tubby promptly. The professor smiled again. “A year is the time it takes the Earth to revolve once around the Sun. Thus you see there are 31,558,149 seconds in a year—which is a little more than 365¼ days.” “Yes,” said Tubby. “I see.” “Now to find out how far light travels in a year we simply take its speed per second and multiply it by sixty. That is its speed per minute. Multiply that by sixty gives it speed per hour and so on up to a day or a year. Do you understand?” “Yes,” said Tubby. “How—how far does it go in a year?” The professor raised his hand. “_That_ is a very simple problem—only mental arithmetic for me. Wait. Let us go further.” “Yes,” said Tubby. “Go on.” “The distance Light travels in one year is called a Light-year. Now, some of the stars are so far away that it takes light many centuries to come from them, traveling at that speed I have just mentioned. Thus we measure their distance from us as so many hundred Light-years. Do you see?” “Yes,” said Tubby. “How—how many miles away from us is the farthest star—the very farthest?” The professor leaped to his feet. “Ah, I was expecting you to ask that.” His face was beaming. “You _are_ a clever man. But I’m ready for you. I figured it all out this afternoon and wrote it down. I’ll show you.” He led Tubby to a corner of the room. Tubby saw a perfectly enormous roll of narrow paper tape, like the tape that comes from a stock ticker, or is rolled up around baby ribbon. Only this roll was bigger than he was. It was hung vertically on a spindle, which had a handle to rotate it so that the tape could be unwound and wound again easily. A few feet away there was a similar spindle, but empty. “I wrote it down for you,” the professor repeated—“the distance in miles of the furthest star from the Earth. I wrote it down—on this.”