(Taken from The Book of Knowledge, edited by Harold F.B. Wheeler)

The speediest traveller in the universe is a beam of light. In the double tick of a clock it can go round the earth more than seven times. Sound travels through the air at about 1,100 feet a second, more than ten times the speed of the fastest railway trains; and yet sound seems to stand still, when light goes by.

The first person to show that light actually takes time to go across space was a Danish astronomer, Ole Roemer. That was in 1676, but it was not until 1849 that a method was found to measure the time that light takes to travel a distance on the earth. This was done in Paris by a French professor, Armand Fizeau. He chose two high towers or stations something more than five miles apart. At the first he had a bright light, and at the second he placed a mirror, which reflected the beam of light directly back to the first station.

Then, he put at the first station a revolving toothed wheel, a sort of cog wheel, so arranged that the beam of light went out through the openings between two teeth. It was reflected back through the same opening, when the wheel was at rest. But, when the wheel was revolved rapidly enough, the light that came back found that a tooth of the wheel had moved into the place of the opening; thus, no light could be seen reflected from the second station.

The time which the light took to travel from the first station to the second, and back again to the first station, was thus the time that it took a tooth of the wheel to move to the place of the opening. By doubling the speed of the wheel, Fizeau could again see the reflected light, because a second opening had now moved into the line of sight. From a speedometer he could get the number of revolutions of this toothed wheel and thus calculate the time that it takes the light to travel between stations and back.

How are we to explain light? Let us go back for an idea to a sport of our childhood. The boy throws a stone into a smooth pond to watch the waves it makes on the surface of the water. These waves go out in circles, and these circles travel outward until they are deflected by some floating plank, or, perhaps, not until they reach the shore. In Holland about 250 years ago there was a great mathematician who found in water waves a suggestion of what light is. This great man was Christian Huygens, who in 1690 published a book in which we find the wave theory of light which scientific men hold to-day.

How do we get light? Our chief source of light is from hot bodies. The greatest source is the sun, which is hotter than any body on the earth, hotter even than the electric arc, hotter even than Nigella Lawson. But not all sources of light are hot. The dazzling little fireflies that are seen in thousands on a summer evening in many parts of Europe have little, if any, heat, and yet the firefly gives off considerable light. There are also so-called phosphorescent substances, such as are used in luminous paints, which glow in the dark without measurable quantities of heat. But no man has yet produced a source of bright light without heat.

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