The stars which form the nebulæ are sometimes grouped so as to form regular shapes, spheres, or more or less lengthened ellipses. Sometimes the sphere is hollow in the centre, and so forms a ring. Nothing more varied, nothing more strange can be imagined than the forms of those nebulæ which have hitherto been examined, and which already number more than a million, of which no two are precisely alike. Certain nebulæ seem to be double, or joined. Others are lengthened out, like serpents, as in that of the _Shield of Sobieski_, represented in figure 9.
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.
[Illustration: 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?
_The Author._ Herschel, having examined the sky of the southern hemisphere from the Cape of Good Hope, in applying his observations to the whole extent of the Milky Way, estimated the number of suns comprised in that immense nebulæ at 18 millions. I have just told you the length of the Milky Way. A ray of light emitted from a star at one of its extremities, and reaching the other, would take 15,000 years to accomplish the transit. So that, when we are looking through the telescope at one of the suns of this nebula, we receive the impression of a ray of light emitted from that star 7000 or 8000 years ago, _i.e._, long before the dawn of the historic ages.[28] The measurement of the Milky Way enables us therefore to measure the extent of other nebulæ, still more distant from us. There are, as I have already said, masses of diffused light in the midst of nebulæ which telescopic analysis has resolved into stars, which are probably much more distant nebulæ. The real distance of these luminous masses can be fixed. If it were asked, to what distance the Milky Way should be removed in order to offer us the aspect of an ordinary nebula, Arago would answer that according to his researches, the Milky Way ought to be removed to a distance equal to 334 times its length. According to this the Milky Way would be seen from the earth at an angle of 10°, and its light would take 5,010,000 years to travel that distance. Thus, light would take _more than five millions of years_ to travel from one of the telescopic nebulæ to our earth. Such are the intervals which exist in the universe, and which our instruments can appreciate. It seems to me that we are now on the borders of the Infinite.