i t The Outline of Science 



an intelligent insect, living on a single leaf in the midst of a 

 Hra/ilian forest, to say what is the shape and size of the 

 . Yi-t man's ingenuity has proved equal to giving an 

 Biwef even to this question, and by a method exactly similar to 

 that which would be adopted by the insect. Suppose, for in- 

 stance, that the forest was shaped as an elongated oval, and the 

 insect lived on a tree near the centre of the oval. If the trees 

 were approximately equally spaced from one another they would 

 appear much denser along the length of the oval than across its 

 width. This is the simple consideration that has guided astrono- 

 . in determining the shape of our stellar universe. There is 

 one direction in the heavens along which the stars appear denser 

 than in the directions at right angles to it. That direction is 

 the direction in which we look towards the Milky Way. If we 

 count the number of stars visible all over the heavens, we find 

 they become more and more numerous as we approach the Milky 

 Way. As we go farther and farther from the Milky Way the 

 stars thin out until they reach a maximum sparseness in direc- 

 tions at right angles to the plane of the Milky Way. We may 

 consider the Milky Way to form, as it were, the equator of our 



in, and the line at right angles to point to the north and 

 south poles. 



Our system, in fact, is shaped something like a lens, and 

 our sun is situated near the centre of this lens. In the remoter 

 part of this lens, near its edge, or possibly outside it altogether, 

 lies the great series of star clouds which make up the Milky 

 Way. All the stars are in motion within this system, but the 

 very remarkable discovery has been made that these motions are 

 not entirely random. The great majority of the stars whose 

 motions can be measured fall into two groups drifting past one 

 another in opposite directions. The velocity of one stream rela- 

 ti\<- to the other is about twenty-five miles per second. The stars 

 forming these two groups are thoroughly well mixed; it is not 

 a case of an inner stream going one way and an outer stream the 



