A VOYAGE IN SPACE
may be grown-ups in the back rows and small people in front: in other words, there may be very bright stars at a distance, and faint ones near to us; and if we do not remember this, our inferences from the counts will be wrong.
Let me give you an illustration. Fig. 86 is a picture of three men. You say unhesitatingly that the big man is near us, the tiny man far away, and the middle man in between. You say this because men are all
nearly of the same height, and when they appear so very different in size as in the picture, the difference must be due to distance. So we think. But now I have deceived you in this picture by omitting certain details; these are not men, but puppets hung on a Christmas tree, as you see in Fig. 87. They are really all at the same distance; and their difference in size is real, because dolls and puppets can be as different in size as we like. What led us wrong was the quiet assumption that