GALACTIC NEBULAE. The second class are generally described as “Galactic Nebulae,” examples being shewn in Plates III, VI (p. 37) and VII (p. 44). These are completely irregular in shape. Their general appearance is that of huge glowing wisps of gas stretching from star to star, and in effect this is pretty much what they are. Like the planetary nebulae, they lie entirely within the galactic system. Even a cursory glance shews that each irregular nebula contains several stars enmeshed with it; minute telescopic examination often extends the dimensions of the nebula almost indefinitely, so that we may have almost the whole of a constellation wrapped up in a single nebula. There is but little doubt as to the physical nature of these nebulae. The space between the stars is not utterly void of matter, but is occupied by a thin cloud of gas of a tenuity which is generally almost beyond description. Here and there this cloud may be denser than usual; here and there again it may be lighted up and made to incandesce by the radiation of the stars within it. In other places it may be entirely opaque to light, lying like a black curtain across the sky. The variations of density, opacity and luminosity in combination produce all the fantastic shapes and varied degrees of light and shade we see in the galactic nebulae. This same opacity is responsible for the dark patches which occur in the general arrangement of the stars. A conspicuous example occurs in the part of the Milky Way shewn in Plate I (p. 23). The dark patch, which looks at first like a hole in the system of stars, is graphically described as “The Coal Sack.” These black patches in the sky cannot represent actual holes, because it is inconceivable that there should be so many empty tunnels through the stars all pointing exactly earthward, so that we are compelled to interpret them as veils of obscuring matter which dim or extinguish the light of the stars behind them. EXTRA-GALACTIC NEBULAE. The third class of nebula is of an altogether different nature. Its members are for the most part of definite and regular shape, and shew various other characteristics which make them easy of identification. They used to be called “white nebulae” from the quality of the light they emitted. Later Lord Rosse’s giant 6-foot telescope revealed that many of them had a spiral structure; these were called “spiral nebulae.” The most conspicuous of all the spiral nebulae is the Great Nebula _M_ 31 in Andromeda, shewn in Plate IV, which is just, and only just, visible to the naked eye. Marius, observing it telescopically in 1612, described it as looking “like a candle-light seen through horn.” Plate V shews a second example, probably of very similar structure, which is viewed from another angle, so as to appear almost exactly edge-on. [Illustration: PLATE IV _Yerkes Observatory_ The Great Nebula _M_ 31 in Andromeda] [Illustration: PLATE V _Mt Wilson Observatory_