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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a9/PSM_V03_D144_Annular_nebula_in_lyra.jpg/300px-PSM_V03_D144_Annular_nebula_in_lyra.jpg)
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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/PSM_V03_D144_Nebula_with_several_rings.jpg/400px-PSM_V03_D144_Nebula_with_several_rings.jpg)
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