Saffman and Taylor showed that including a surface tension-induced pressure difference across the interface led to a modification of the stability criterion: in this case instead of being unstable to all wavelengths, surface tension stabilizes perturbations of small wavelength and overall the interface is unstable to disturbances of wavelength greater than
is maximized with respect to n, i.e. the perturbation grows
fastest, when the wavelength is 
. In practice,
this is the disturbance which will be observed as the instability
develops.