re_ than sixty billions of miles.
Struve did not contradict this--nay, he certainly confirmed it--when he
showed that the distance could not be more than forty billions.

Nearly half a century has elapsed since Struve made his observations.
Those observations have certainly been challenged; but they are, on the
whole, confirmed by other investigations. In a critical review of the
subject Auwers showed that Struve's determination is worthy of
considerable confidence. Yet, notwithstanding this authoritative
announcement, the study of 61 Cygni has been repeatedly resumed. Dr.
Bruennow, when Astronomer Royal of Ireland, commenced a series of
observations on the parallax of 61 Cygni, which were continued and
completed by the present writer, his successor. Bruennow chose a fourth
comparison star (marked on the diagram), different from any of those
which had been used by the earlier observers. The method of observing
which Bruennow employed was quite different from that of Struve, though
the filar micrometer was used in both cases. Bruennow sought to determine
the parallactic ellipse by measuring the difference in declination
between 61 Cygni and the comparison star.[38] In the course of a year it
is found that the difference in declination undergoes a periodic change,
and from that change the parallactic ellipse can be computed. In the
first series of observations I measured the difference of declination
between the preceding star of 61 Cygni and the comparison star; in the
second series I took the other component of 61 Cygni and the same
comparison star. We had thus two completely independent determinations
of the parallax resulting from two years' work. The first of these makes
the distance forty billions of miles, and the second makes it almost
exactly the same. There can be no doubt that this work supports Struve's
determination in correction of Bessel's, and therefore we may perhaps
sum up the present state of our knowledge of this question by saying
that the distance of 61 Cygni is much nearer to the forty billions of
miles which Struve found than to the sixty billions which Bessel
found.[39]

It is desirable to give the reader the means of forming his own opinion
as to the quality of the evidence which is available in such researches.
The diagram in Fig. 95 here shown has been constructed with this object.
It is intended to illustrate the second series of observations of
difference of declination which I made at Dunsi