288 
IONISATION, DIFFUSION, ROTATION 
Von Zeipel has also shown that the law H oc g applies when the star is 
distorted by tidal forces. 
In (200-2) the approximation for e is used in a more specialised way 
than in a discussion of the total radiation of the star, and it would seem 
necessary to examine how closely the result is bound up with the accuracy 
of the approximation before we can be sure that it will apply to actual 
stars. I daresay it will be found that the approximation still justifies 
itself, but it is not at all obvious that it is legitimate. The distribution of 
surface brightness over a tidally disturbed star is of considerable practical 
importance in the interpretation of the light-curves of eclipsing variables.