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78 Recent Advances in Stellar Astronomy
bright fringes produced by one star’s light will just occupy
the dark spaces between the fringes given by the other,
and the system of fringes will disappear. This happens
when the components of the double star are of the same
magnitude. If one of the stars is slightly brighter than
the other, the “visibility” of the fringes diminishes, and
they assume a washed-out appearance, without vanishing
entirely. In either case, the existence of a double star can
be detected, although the components are separated by less
than half the distance at which their spurious images would
begin to be confused, if observed with the full aperture.
By making the distance between the apertures adjustable,
and arranging them so that the line joining them can be
turned to cross the main aperture in any direction, it is
possible to measure the distance of a close double star
(changing the distance of the slits until the fringes vanish).
In this way measures were made upon the bright star
Capella, which has long been known, from spectroscopic
measures, to be a very close binary, with a period of 104
days. No telescope has ever resolved the components,
but, with the interferometer, immediate evidence of the
duplicity was secured. What is more, and better, it was
found that very precise measures could be made, follow-
ing an ingenious method devised by Anderson, although
the distance was as small as 0".04. Indeed, the precision
of the results appears to be fully twenty times greater
(as regards the distance measures, at least) than can be
secured by any method of observation previously known.
Best of all, it was found that atmospheric disturbances—
contrary to expectation—had much less influence upon the
definition of the fringes than upon that of the star-image
as a whole. This is conspicuous even to the novice who
has the privilege of observing with the instrument. While

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