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 scattered. 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 definite 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.
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