A VOYAGE IN SPACE
Perhaps you don't know that last colour; it is mainly put in to fill up the scale, which would otherwise stop awkwardly short; it does not mean ultra-marine, but ultra-violet. I don't mean that the colours correspond to these notes in detail, but that in passing from red to violet the pitch rises.
If a star is whistling a green note, that is to say sending us green light, then if it is also coming directly at us the green note will rise in the scale towards the blue; if the star is running away the green note will fall in the scale towards the yellow. How much it rises or falls depends on the rate of movement, and I may say at once that the rise or fall is very slight indeed, because however quick the movement of the star may seem to us, it is very, very slow compared with the enormous velocity of light. But the rise or fall is noticeable and measurable; by noticing it and measuring it astronomers have found out which stars are coming at us and which are running away, and how fast they are coming or going.
Some of the best illustrations of this way of measuring velocities, whether by sound or by light, are afforded by bodies which are whirling or turning round, because sometimes they approach, sometimes they recede. Here is an instrument called a "bull-roarer" which some of you may have made for yourselves a shaped piece of wood with a string tied to it. If I whirl it round it makes a humming noise. Now I can whirl it round so that it remains always at the same distance from you, and then the hum is steady; but if I turn end-on to you, sometimes it approaches and sometimes