THE EXTENT OF THE UNIVERSE 329 



One of the great problems connected with the universe is 

 that of its possible extent. How far away are the stars? 

 One of the unities which we have described leads at once to 

 the conclusion that the stars must be at very different 

 distances from us; probably the more distant ones are a 

 thousand times as far as the nearest; possibly even farther 

 than this. This conclusion may, in the first place, be based on 

 the fact that the stars seem to be scattered equally through- 

 out those regions of the universe which are not connected 

 with the Milky Way. To illustrate the principle, suppose a 

 farmer to sow a wheat-field of entirely unknown extent with 

 ten bushels of wheat. We visit the field and wish to have 

 some idea of its acreage. We may do this if we know how 

 many grains of wheat there are in the ten bushels. Then 

 we examine a space two or three feet square in any part of 

 the field and count the number of grains in that space. If 

 the wheat is equally scattered over the whole field, we find 

 its extent by the simple rule that the size of the field bears 

 the same proportion to the size of the space in which the 

 count was made that the whole number of grains in the ten 

 bushels sown bears to the number of grains counted. If 

 we find ten grains in a square foot, we know that the number 

 of square feet in the whole field is one-tenth that of the 

 number of grains sown. So it is with the universe of stars. 

 If the latter are sown equally through space, the extent of 

 the space occupied must be proportional to the number of 

 stars which it contains. 



But this consideration does not tell us anything about the 

 actual distance of the stars or how thickly they may be scat- 

 tered. To do this we must be able to determine the distance 

 of a certain number of stars, just as we suppose the farmer 

 to count the grains in a certain small extent of his wheat- 

 field. There is only one way in which we can make a defi- 

 nite measure of the distance of any one star. As the earth 

 swings through its vast annual circuit round the sun, 

 the direction of the stars must appear to be a little different 

 when seen from one extremity of the circuit than when seen 

 from the other. This difference is called the parallax of the 

 stars; and the problem of measuring it is one of the most 

 delicate and difficult in the whole field of practical astronomy. 



