an accumulation of matter toward the centre, with a gradual increase of density, and a rotatory movement. But, if we combine with this motion the supposition of an opposing medium, it is difficult to harmonize such a system with the known laws of statics. Accurate measures are, therefore, of the highest interest for the purpose of showing
Fig. 7.
Annular Nebula in Lyra.
whether actual rotation or other changes are taking place in these nebulæ; but, unfortunately, they are rendered extremely difficult and uncertain by the want of outline, and by the remarkable faintness of these nebulous objects.
Fig. 8.
Nebula with Several Rings.
The transition state from the spiral to the annular form is shown in such nebulae as the one represented in Fig. 6; and they then pass
