is a picture of a strip of nebula which seems curiously split down the middle. Here, again, we must remember that the nebula is not a flat object like a piece of paper, but a bulky object like a cushion. If here is a split, it must be a slice right through the cushion, and, besides this, the slice must be straight in the direction of the Earth. Both these difficult
Fig. 83. Split Nebula in Andromeda.
suppositions are unnecessary if we suppose there is a dark streak in front of the nebula; we may go further and presume that the streak is part of a flat ring surrounding the nebula, stopping the light where it passes in front.
Fig. 84 is a picture of a nebula of a different kind. You see a gauzy mass running down the picture with a bright edge: and you will see that the stars are much more numerous on one side of the
