Lord Rosse was the first to discover that curious disposition of the nebulæ called spiral.
Such a form is inexplicable, but it is certain that the suns which compose the nebulæ are often grouped, not around a centre, not in shapeless heaps, but in regular curves, on a system which seems to reveal the existence of some mysterious force acting upon those stars, which are distributed along lines representing spirals of different diameter.
Fig. 9.—The Shield of Sobieski.
In speaking of the stars, I have said that there are coloured stars or suns. I will add here that nebulæ are observed coloured red, green, and yellow, which is an additional proof that they are only agglomerations of stars. That immense semi-luminous band which traverses the celestial vault, girding it with a silver belt, is not, as it was long supposed to be, a diffused quantity of luminous matter. The telescopic analysis of the Milky Way shows that it consists of a long series of nebulæ. The length of the Milky Way is from 700 to 800 times the distance from Sirius to the sun, a distance which is 1,373,000 times that from the earth to the sun.[27]
Theophilus. Can any idea be formed of the number of stars comprised in the Milky Way?
Fig. 10.—The Milky Way.