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. 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.

The nineteenth century was well on its way before the instruments
of the astronomer were brought to such perfection as to admit of
the measurement. From the time of Copernicus to that of Bessel
many attempts had been made to measure the parallax of the stars,
and more than once had some eager astronomer thought himself
successful. But subsequent investigation always showed that he had
been mistaken, and that what he thought was the effect of parallax