slower than the inner ones. Hence the ends of the line, instead of being more displaced, are less displaced, and we get an elongated Z shape VXYW. This was predicted from knowledge of the movements, and when the spectroscope was actually turned on to Saturn the prediction was beautifully verified! So that we may feel all confidence in
Fig. 89.
the power of the spectroscope to tell us about these movements of approach and recession.
We have, then, two different ways of measuring the movements of the stars; it is a piece of great good luck for us that we have two, for we can often test what one method has told us by using the other. And now I want to tell you some of the things we have found out by watching the movements of the stars in these two ways.
First of all let us pay a few visits to the double stars—cases where two stars are seen close together, and where careful watching has shown that they are revolving round each other. Sometimes one is so much the larger that we may almost say that the smaller is revolving round it, like our Earth round the Sun; but there are other cases when the stars
