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but that does not deceive us into thinking that this room full of people is flat. We know that there are some in the front row who come out big in the picture and some in the back rows who come out smaller. And so in the case of the stars; big bright ones are probably in the front row near the Earth: the very faint ones in the back row. I say probably because we cannot be quite sure. Even in this room there are some very small people in the front row; and they come out small in the picture; while their grown-up parents may look large in the picture, in spite of the fact that they are in a row behind. In this case we can generally tell from the dress, or the character in the face, that the difference of size is due to age and not to the position in the room; but the stars do not help us in this way by their appearance, so that we are liable to mistakes if we assign their distances from us simply by the brightness.

But now what are we to think of the places on the photograph where there seem to be no stars? There are some directions in which I can see no people in this room, namely, when I look along the corridors: but I have to be careful to stand in one special place (A) to see this emptiness. If I move to the right, as to B, then I lose the appearance of vacancy in all three corridors (Fig. 82).

You can try a similar thing for yourselves with an empty tube. If you point it straight to your eye you can see the vacant space at the far end; but if you incline the tube, the far end disappears behind one of the sides. If the tube is short, you may have to incline it a good deal before the end disappears;