16. But indeed the nebulæ themselves, and especially the most remote of the nebulæ, or at least those which most especially require the most powerful telescopes, offer far more decisive proofs that their resolvability or non-resolvability,--their apparent constitution as diffused and vaporous masses,--does not depend upon their distance. A remarkable fact in the irregular, and in some of the regular nebulæ[4] is, that they consist of long patches and streaks, which stretch out in various directions, and of which the form[5] and extent vary according to the visual power which is applied to them. Many of the nebulæ and especially of the fainter ones, entirely change their form with the optical power of the instrument by which they are scrutinized; so that, as seen in the mightier telescopes of modern times, the astronomer scarcely recognizes the figures in which the earlier observers have recorded what they saw in the same place. Parts which, before, were separate, are connected by thin bridges of light which are now detected; and where the nebulous space appeared to be bounded, it sends off long tails of faint light into the surrounding space. Now, no one can suppose that these newly-seen portions of the nebula are immensely further off than the other parts. However little we know of the nature of the object, we must suppose it to be one connected object, with all its parts, as to sense, at the same distance from us. Whether therefore it be resolvable or no, there must be some other reason, besides the difference of distance, why the brighter parts were seen, while the fainter parts were not. The obvious reason is, that the latter were not seen because they were thin films which required more light to see them. We are led, irresistibly as it seems, to regard the whole mass of such a nebula, as an aggregation of vaporous rolls and streaks, assuming such forms as thin volumes of smoke or vapor often assume in our atmosphere, and assuming, like them, different shapes according to the quantity of light which comes to us from them. If, as soon as one of these new filaments or webs of a nebula comes into view, we should say, Here we have a new array of suns and of worlds, we should judge as fantastically, as any one who should combine the like imaginations with the varying cloud-work of a summer-sky. To suppose that all the varied streaks by which the patch of nebulous light shades off into the surrounding darkness, and which change their form and extent with every additional polish which we can give to a reflecting or refracting surface, disclose, with every new streak, new worlds, is a wanton indulgence of fancy, to which astronomy gives us no countenance.[6]