appears to
me to be the most dangerous assumption involved in the whole theory.

Making these assumptions, however, it is found that if each of the
eclipsing stars were spherical it would not be possible to generate such
a curve with the closest accuracy. The two stars are certainly close
together, and it is obvious that in such a case the tidal forces
exercised by each on the other must be such as to elongate the figure
of each towards the other. Accordingly it is reasonable to adopt the
hypothesis that the system consists of a pair of elongated ellipsoids,
with their longest axes pointed towards one another. No supposition
is adopted a priori as to the ratio of the two masses, or as to their
relative size or brightness, and the orbit may have any degree of
eccentricity. These last are all to be determined from the nature of the
light-curve.

In the case of RR Centauri, however, Dr Roberts finds the conditions are
best satisfied by supposing the orbit to be circular, and the sizes and
masses of the components to be equal, while their luminosities are to
one another in the ratio of 4 to 3. As to their shapes he finds them
to be so much elongated that they overlap, as exhibited in his figure
titled "The shape of the star RR Centauri" (Fig. 6.). The dotted curve
shows a form of equilibrium of rotating liquid as computed by me some
years before, and it was added for the sake of comparison.

On turning back to Fig. 5 the reader will see in the smooth dotted curve
the light variation which would be exhibited by such a binary system as
this. The curve is the result of computation and it is impossible not to
be struck by the closeness of the coincidence with the series of black
dots which denote the observations.

It is virtually certain that RR Centauri is a case of an eclipsing
binary system, and that the two stars are close together. It is not
of course proved that the figures of the stars are ellipsoids, but
gravitation must deform them into a pair of elongated bodies, and, on
the assumptions that they are not enveloped in an absorptive atmosphere
and that they are ellipsoidal, their shapes must be as shown in the
figure.

This light-curve gives an excellent illustration of what we have reason
to believe to be a stage in the evolution of stars, when a single star
is proceeding to separate into a binary one.

As the star is faint, there is as yet no direct spectroscopic evidence
of orbital motion. Let us turn therefo