It has already been noticed how the “great nebulae” form what Herschel described as a system of “island-universes,” distinct and detached both from one another and from the galactic system of stars. Hubble has found that these nebulae are all of comparable size, being, as fig. 2 (p. 15) has shewn, of size comparable with, although smaller than, the galactic system.

This of itself would encourage the conjecture that the great nebulae may be star-clouds, of the same general nature as the cloud of stars surrounding the sun. This view of the nature of the great nebulae has been very prevalent since the time of the Herschels, and various items of recently gained knowledge appear to give it support rather than the reverse.

Viewed from a fairly remote nebula, our galactic system of stars would appear as a cloud of faint light, which telescopes of terrestrial power would be unable to resolve into separate stars. Since the average light from these stars gives a spectrum of F or G type, the composite spectrum of this cloud of stars would closely resemble a stellar spectrum of F or G type, and this is precisely the type of spectrum shewn by the great nebulae, their spectra even being crossed by dark lines of the same general character as the Fraunhofer lines in the solar spectrum.