nk. Each of the dots
represents one night's observations. The height of the dot is the
observed difference of declination between 61 (B) Cygni and the
comparison star. The distance along the horizontal line--or the
abscissa, as a mathematician would call it--represents the date. These
observations are grouped more or less regularly in the vicinity of a
certain curve. That curve expresses where the observations should have
been, had they been absolutely perfect. The distances between the dots
and the curve may be regarded as the errors which have been committed in
making the observations.

[Illustration: Fig. 95.--Parallax in Declination of 61 Cygni.]

Perhaps it will be thought that in many cases these errors appear to
have attained very undesirable dimensions. Let us, therefore, hasten to
say that it was precisely for the purpose of setting forth these errors
that this diagram has been shown; we have to exhibit the weakness of the
case no less than its strength. The errors of the observations are not,
however, intrinsically so great as might at first sight be imagined. To
perceive this, it is only necessary to interpret the scale on which this
diagram has been drawn by comparison with familiar standards. The
distance from the very top of the curve to the horizontal line denotes
an angle of only four-tenths of a second. This is about the apparent
diameter of a penny-piece at a distance of _ten miles_! We can now
appraise the true magnitude of the errors which have been made. It will
be noticed that no one of the dots is distant from the curve by much
more than half of the height of the curve. It thus appears that the
greatest error in the whole series of observations amounts to but two or
three tenths of a second. This is equivalent to our having pointed the
telescope to the upper edge of a penny-piece fifteen or twenty miles
off, instead of to the lower edge. This is not a great blunder. A rifle
team whose errors in pointing were more than a hundred times as great
might still easily win every prize at Bisley.

We have entered into the history of 61 Cygni with some detail, because
it is the star whose distance has been most studied. We do not say that
61 Cygni is the nearest of all the stars; it would, indeed, be very rash
to assert that any particular star was the nearest of all the countless
millions in the heavenly host. We certainly know one star which seems
nearer than 61 Cygni; it lies in one of the southern con