apart. The apparent meeting is an effect of "perspective." Some of you have no doubt learnt perspective drawing and you know that parallel lines have to be drawn to meet in a "vanishing point." So Mr. Boss's group of stars may be moving along parallel lines, without closing up: and indeed we can find out that this is the fact,
by using the second method of watching their movements which we noticed, with the spectroscope. The "vanishing point" represents the direction in the far distance to which they are migrating, like a flock of birds such as we see in Fig. 92.
There is one more group which I should like you to know of, part of which was also identified long ago by Mr, Proctor. He pointed out that if we