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 throughout 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 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 tha