rom the fluctuating and tiresome disturbances of
atmospheric refraction. The reason bright stars were chosen was because
they were presumably nearer than the others; and indeed a rough guess at
their probable distance was made by supposing them to be of the same
size as the sun, and estimating their light in comparison with sunlight.
By this confessedly unsatisfactory method it had been estimated that
Sirius must be 140,000 times further away than the sun is, if he be
equally big. We now know that Sirius is much further off than this; and
accordingly that he is much brighter, perhaps sixty times as bright,
though not necessarily sixty times as big, as our sun. But even
supposing him of the same light-giving power as the sun, his parallax
was estimated as 1".8, a quantity very difficult to be sure of in any
absolute determination.

Relative methods were, however, also employed, and the advantages of one
of these (which seems to have been suggested by Galileo) so impressed
themselves upon William Herschel that he made a serious attempt to
compass the problem by its means. The method was to take two stars in
the same telescopic field and carefully to estimate their apparent
angular distance from each other at different seasons of the year. All
such disturbances as precession, aberration, nutation, refraction, and
the like, would affect them both equally, and could thus be eliminated.
If they were at the same distance from the solar system, relative
parallax would, indeed, also be eliminated; but if, as was probable,
they were at different distances, then they would apparently shift
relatively to one another, and the amount of shift, if it could be
observed, would measure, not indeed the distance of either from the
earth, but their distance from each other. And this at any rate would be
a step. It might be completed by similarly treating other stars in the
same field, taking them in pairs together. A bright and a faint star
would naturally be suitable, because their distances were likely to be
unequal; and so Herschel fixed upon a number of doublets which he knew
of, containing one bright and one faint component. For up to that time
it had been supposed that such grouping in occasional pairs or triplets
was chance coincidence, the two being optically foreshortened together,
but having no real connection or proximity. Herschel failed in what he
was looking for, but instead of that he discovered the real connection
of a numbe