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it recedes, and you can hear the note change up and down.

Similarly, if one star is whirling round another, it may do so in such a way as to remain nearly at the same distance from us, when its light remains of uniform colour; but the orbit may also be end-on to us, when we see the colours go up and down. I am using the word colour in a special sense: when we spread out the light of a star into colours by a spectroscope, we see the band crossed by a number of dark lines which mean special colours: and it is these lines which are displaced towards the violet or towards the red (see Plate facing p. 273). And now we will look at a very beautiful instance of this displacement in the case of the planet Saturn and its ring.

Let us first consider the planet himself or the "ball," as it is usually called to distinguish it from the "ring." Suppose we have cut a slice right through the centre O; the slice will be turning round O (Fig. 89). The left side A is coming at us and the right side B is running away; so that if we pass the light of this slice through a spectroscope, one end of the colour-line will be displaced one way and the other the other, in consequence of which we get a sloped line XY instead of a horizontal one PQ. Next, if the ring were solid and attached to the ball, the same state of things would continue in the ring; the line XY would be merely extended to HK. But the ring is not solid, as we have seen: it is made up of little satellites, the outer ones moving, not faster than the inner ones, as they would if the ring were solid, but