r smaller, and at that enormous
distance the fact whether it advances or recedes makes no difference in
its size. Now, the spectroscope has changed all this, and we can tell
quite as certainly if a star is coming toward us as we can if it moves
to one side. I will try to explain this. You know, perhaps, that sound
is caused by vibration in the air. The noise, whatever it is, jars the
air and the vibrations strike on our ears. It is rather the same thing
as the result of throwing a stone into a pond: from the centre of the
splash little wavelets run out in ever-widening circles; so through the
air run ever-widening vibrations from every sound. The more vibrations
there are in a second the shriller is the note they make. In a high note
the air-vibrations follow one another fast, pouring into one's ear at a
terrific speed, so that the apparatus in the ear which receives them
itself vibrates fiercely and records a high note, while a lower note
brings fewer and slower vibrations in a second, and the ear is not so
much disturbed. Have you ever noticed that if a railway engine is
sweeping-toward you and screaming all the time, its note seems to get
shriller and shriller? That is because the engine, in advancing, sends
the vibrations out nearer to you, so more of them come in a second, and
thus they are crowded up closer together, and are higher and higher.

Now, light is also caused by waves, but they are not the same as sound
waves. Light travels without air, whereas sound we know cannot travel
without air, and is ever so much slower, and altogether a grosser,
clumsier thing than light. But yet the waves or rays which make light
correspond in some ways to the vibrations of sound. What corresponds to
the treble on the piano is the blue end of the spectrum in light, and
the bass is the red end. Now, when we are looking at the spectrum of
any body which is advancing swiftly toward us, something of the same
effect is observed as in the case of the shrieking engine. Take any star
and imagine that that star is hastening toward us at a pace of three
hundred miles a second, which is not at all an unusual rate for a star;
then, if we examine the band of light, the spectrum, of such a star, we
shall observe an extraordinary fact--all these little lines we have
spoken of are shoved up toward the treble or blue end of the spectrum.
They still remain just the same distances from each other, and are in
twos and threes or single, so that the