Sir William Herschel believed he had confirmed his own view of the structure of the galactic system in the same way, by discovering similar systems, of which he could obtain a bird’s-eye view because they were entirely extraneous to the galaxy. He spoke of these objects as “island universes” and believed them to be clouds of stars. They were of hazy nebular appearance, and although it was impossible to distinguish the separate stars in them, he believed that sufficient telescopic power would make this possible, just as it had enabled Galileo to see the separate stars in the Milky Way. These objects, which we shall describe almost immediately, are generally known as “extra-galactic nebulae“ from their position, although we shall frequently find it convenient to use the briefer term “great nebulae,” to which their immense size fully entitles them. NEBULAE A telescope exhibits a planet as a disc of appreciable size, and an eye-piece which magnifies 60 times will make Jupiter look as large as the moon. Yet an eye-piece which magnifies 60 times, or any greater number of times, can never make a star look as large as the moon. No magnification within our command causes any star to appear as anything other than a mere point of light. The stars are of course enormously larger than Jupiter, but they are also enormously more distant, and it is the distance that wins. [Illustration: PLATE II N.G.C. 2022 N.G.C. 6720 N.G.C. 1501 N.G.C. 7662 _Mt Wilson Observatory_ Planetary Nebulae] [Illustration: PLATE III _Mt Wilson Observatory_ The Nebula in Cygnus] The telescope nevertheless shews a number of objects which appear bigger than mere points of light. They are generally of a faint, hazy appearance, and so have received the general name of “nebulae.” Detailed investigation has shewn that these fall into three distinct classes. PLANETARY NEBULAE. The first class are generally described as “Planetary Nebulae.” There is nothing of a planetary nature about them beyond the fact that, like the planets, they shew as finite discs in a telescope. A few hundreds only of these objects are known, four typical examples being illustrated in Plate II. They all lie within the galactic system. We shall discuss their physical structure below (p. 321). For the moment, it is enough to say that they are probably of the nature of stars which have in some way become surrounded by luminous atmospheres of enormous extent. If so they of course disprove our general statement that no star ever appears as anything but a point of light in a telescope; we must make an exception in favour of the planetary nebulae.