94 The Realm of Nature CHAP. 



in three long stages with a slight shift of position in each. 

 Each star thus prints itself as a little triangle of three points, 

 while in consequence of its relative motion an asteroid pre- 

 sents its record in one little blurred streak and can thus be 

 readily detected. 



140. Form of the Universe. On a clear moonless 

 night a luminous gauzy band called the Milky Way may be 

 seen spanning the sky like a wide but ragged and colourless 

 rainbow. As this is visible from all parts of the Earth it 

 evidently forms a complete girdle round the star-dome. A 

 telescope of moderate power shows that the Milky Way is 

 really a dense pavement of stars. There is no reason to 

 believe that any two of these stars are nearer each other 

 than the Sun and a, Centauri, and the apparent crowding 

 is simply an optical effect due to their great number. If 

 we were led blindfolded into a regular pine plantation, and 

 on looking round found that to east and west the tree trunks 

 stood out sharply against the sky, affording a glimpse of 

 diversified country beyond them, while to north and south 

 the trunks were crowded so closely that they formed merely 

 a reddish mist under the dark green leafage, we would natur- 

 ally conclude that the wood was planted in a long narrow 

 belt running north and south. So from our station in the Uni- 

 verse the Milky Way appears as the direction in which the 

 extent of star-sown space is greatest ; the widely strewn stars 

 indicate the sides on which it is least. The form of the 

 Universe, if this mode of reasoning be correct, is that of 

 a vast disc, the edge of which, as shown by a division in 

 the Milky Way, is partially split and bent back. Within 

 this expanse the great family of 100,000,000 or more stars is 

 supposed to be clustered together, and separated by incalcul- 

 able distances of vacancy from other universes which may 

 exist. 



141. Star-clusters. As one might catch glimpses of 

 other forests through the tree trunks on either side of the 

 long plantation, so we catch glimpses of remote universes 

 through the thinly star -sown regions remote from the 

 Milky Way. These are faint patches of light, which were 

 long called Nebula from their cloudy appearance. Genera- 



