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Rh 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 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