The Sizes and Masses of the Stars 79
the star images dance about, the fringes remain almost
fixed, and present a singular appearance of objective reality.
So far we have spoken as if the single stars of the pair
were mere luminous points. If the interferometer is turned
upon an object of finite angular diameter, and the distance
between the apertures increased, the fringes gradually fade
away and finally disappear. How this happens may be
seen by imagining the star-disk to be divided into two semi-
circles by a line perpendicular to the position angle of the
interferometer apertures. Each half-disk is an independent
source of light, and will give its own system of fringes.
As the apertures are separated and the fringes become
narrower the two sets will finally become superposed,
giving a uniform illumination. When this happens, the
reading of our instrument (assuming that we interpret our
observations as in the case of a double star) will give the
distance between the centre of the luminous area of one
semi-circle and that of the other. It is easy to see that
this distance will be rather less than half the diameter of
the whole circle—in fact it is about forty-one per cent, of
it. Our readings may, therefore, be calibrated to give at
once the diameter of the star-disk.
The resolving power of the interferometer is less in this
case than for a double star, and a greater base line than
even the diameter of the 100-inch telescope was desirable.
An auxiliary apparatus was therefore adopted, consisting
of a rigid steel beam twenty feet long, fixed across the
outer end of the telescope, carrying two mirrors inclined
at 45° to the axis of the beam, and capable to being set
at any desired 'distance apart, up to the full light of the
beam. These reflect the star’s light inward along the beam
to another pair of diagonal mirrors, which send it into the
telescope. With this device the effective distance between