(. Abbe—Nebule of Herschel’s Catalogue. 43 
very few double nebule, nor can we for a long time hope to 
determine the planes of the orbits of any binary nebula, if such 
exist; on the other hand, in regard to the axes of rotation of 
nebulze or the planes of their equators, there is more room for 
study. Itisa plausible hypothesis that some nebule are in 
rotation about their respective axes, and only in the case of an 
irregular nebula do we find presumptive evidence of numerous 
centers of aggregation and rotation ; this latter class will not 
now further claim our attention. Those nebule whose whole 
regard to them it may be remarked that if these nebule are 
gaseous and without rotation, we can only explain their apparent 
shape by supposing them to be endowed with a motion of trans- 
lation, to be in fact wisps, like comet’s tails; if, on the other 
hand, they be in a state of rotation, they must be flat rings or 
dises or extended flattened ellipsoids, and we are authorized to 
consider that the planes of their equators do sensibly pass 
through the position of the observer; similarly, if the nebulous 
appearance be due to the presence of lenticular or ring-shaped 
Cloud of asteroids, or of meteoric dust, we shall be able to make 
a determination of the plane of the orbits of these bodies. 
_ the trigonometrical formule by which we may compute the 
night ascension and declination of the poles of a very much 
extended nebula are given in the following note 
¥ ascertained by means of the differential formule. 
