uniform ; for though in some parts their light is uniformly milky, in
others it is more faint, and might, at first sight, be thought resolvable into stars. The greater brightness in one part than in another
may arise either from greater depth or from greater density, and is
thought by Dr. Herschel rather to arise from the latter, which he as-
cribes to a mutual gravitation of its parts to each other. Facts, says
the author, are not wanting to prove that such a power has been exerted; and it is his intention to point out a series of phenomena
where the vestiges of such exertion may be distinctly seen.
Those nebulae, which from their present want of uniformity seem to have more than one centre of attraction, may, in fact, at this time consist of two nebulae, the limits of which unite only in appearance; or they may be actually united, but undergoing a gradual separation.
In the same manner other nebulae, for which the author refers to his catalogues, are found to be treble, quadruple, sextuple, &c.
With regard to the forms of nebulae, some are narrow and very long. others of equal length are in a certain degree broader; others again are of an irregular figure; and from all these, nothing can be inferred as to their extent in the third dimension, which is not seen. But when the visible surface is nearly elliptical, and still more if the circumference be nearly circular there, a spheroidical or spherical form may with some confidence be presumed; and these, it is ob- served, are forms naturally resulting from a principle of attraction.
The central brightness of some nebulae point out the seat of prin- cipal attraction; and though in many such a difference is scarcely perceptible, there are many others in which the brightness of the centre is considerably greater than that of their circumference, so as to present very much the appearance of many telescopic comets, of which the nucleus is not distinct; and others also, in which a cen- tral nucleus is distinctly seen, surrounded by a bright and circular nebula.
These various degrees of condensation, seen in different nebulae, are supposed by the author successively to take place in the same nebula, and are thus connected into one view, to show the progress of condensation.
The condensation, says Dr. Herschel, may be considered as evi- dence of the'n- materiality; and he also infers opacity, which is an- other property of matter, from the circumstance of their not being so bright as he would expect from their observed condensation.
The appearance of certain very regular nebulae, with extensive branches, suggests to Dr. Herschel various queries respecting the gradual change and ultimate result of condensation.
Do not the branches connected with a nucleus resemble the zodiacal light connected with our sun?
May not portions of branches collect into a planetary form and revolve around the central nucleus, having themselves a rotatory motion, in consequence of the inequality and irregular position of different branches P
