It can be talking something about soap bubble producing nonuniform interence pattern. It is because the thickness of upper region of soap bubble is very thin with path differnece amlost zero due to pull by gravity.
The nonuniform thickness results in nonuniform fringes.
In essence, the relationship can be described by the following equation.
$\displaystyle 2nt = m\lambda$ or $\displaystyle 2nt=(k0.5)\lambda$
where $\displaystyle t$ is the thickness of film of material between
$\displaystyle \lambda$ being the wavelength
n ,m and k being constants
