558 The Genesis of Double Stars 
smooth dotted curve will be explained hereafter, but, by reference 
to the scale of magnitudes on the margins of the figure, it may 
be used to note that the dots might be brought into a perfectly 
smooth curve by shifting some few of the dots by about a hundredth 
of a magnitude. 
This light-curve presents those characteristics which are due 
to successive eclipses, but the exact form of the curve must depend 
on the nature of the two mutually eclipsing stars. If we are to inter- 
pret the curve with all possible completeness, it is necessary to make 
certain assumptions as to the stars. It is assumed then that the 
stars are_equally bright all over their disks, and secondly that they 
are not surrounded by an extensive absorptive atmosphere. This last 
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 
Fig. 6. 
The shape of the star RR Centauri. 
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 & 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 now reproduced as Fig. 6. The dotted curve 
