must be a certain number, at any rate, which swing backwards and forwards through the centre, and though these are mixed with others which swing wide, there is still enough oscillation to enable Professor Kapteyn to make his great discovery. I must not, however, mislead you into thinking that he agrees with my interpretation of it, for he does not. He prefers to regard the movements as parallel.
There is another point to be noticed about the stars which circle round the centre of the cluster instead of passing to and fro through it or near it. They remain at a distance from the centre, possibly at a great distance; if so they may form a separate ring round the central cluster. This might explain the Milky Way, that beautiful soft track of light which you can see crossing the sky when there is not too much moonlight. It makes a complete ring round us, though we cannot realize that without going into the southern hemisphere as well as the northern: and when we look at it through a powerful telescope, or take photographs of it, we find that it is dotted over with thousands and thousands of stars. Perhaps it is a ring of the stars which always keep far away from the centre, but as yet we cannot tell, because we have not sufficient knowledge of the movements of these stars. To measure the movement of a star we must record its position carefully at some selected time, then wait a number of years and note how far it has changed. If the star is far away, the change will be very slow and we must wait proportionately longer in order to detect it. For the stars near us a few years may suffice: there are one or two