ars have been
induced to disclose the secret of their distance. We are able to give
some answer to the question--How far are the stars? though it must be
confessed that our reply up to the present moment is both hesitating and
imperfect. Even the little knowledge which has been gained possesses
interest and importance. As often happens in similar cases, the
discovery of the distance of a star was made independently about the
same time by two or three astronomers. The name of Bessel stands out
conspicuously in this memorable chapter of astronomy. Bessel proved
(1840) that the distance of the star known as 61 Cygni was a measurable
quantity. His demonstration possessed such unanswerable logic that
universal assent could not be withheld. Almost simultaneously with the
classical labours of Bessel we have Struve's measurement of the distance
of Vega, and Henderson's determination of the distance of the southern
star a Centauri. Great interest was excited in the astronomical
world by these discoveries, and the Royal Astronomical Society awarded
its gold medal to Bessel. It appropriately devolved on Sir John Herschel
to deliver the address on the occasion of the presentation of the medal:
that address is a most eloquent tribute to the labours of the three
astronomers. We cannot resist quoting the few lines in which Sir John
said:--

"Gentlemen of the Royal Astronomical Society,--I congratulate you
and myself that we have lived to see the great and hitherto
impassable barrier to our excursion into the sidereal universe,
that barrier against which we have chafed so long and so
vainly--_aestuantes angusto limite mundi_--almost simultaneously
overleaped at three different points. It is the greatest and most
glorious triumph which practical astronomy has ever witnessed.
Perhaps I ought not to speak so strongly; perhaps I should hold
some reserve in favour of the bare possibility that it may be all
an illusion, and that future researches, as they have repeatedly
before, so may now fail to substantiate this noble result. But I
confess myself unequal to such prudence under such excitement. Let
us rather accept the joyful omens of the time, and trust that, as
the barrier has begun to yield, it will speedily be effectually
prostrated."

Before proceeding further, it will be convenient to explain briefly how
the distance of a star can be measured. The problem is