The typical light microscope at its maximum magnification power has a resolution of 200 nm. The light microscope is able to view and distinguish between two lines or points that are no less than 200 nm apart. Since our membrane of interest is 8 nm apart, it won't be visible under the light microscope. Several such membranes are needed to lined side by side to make the viewing possible. The number of membranes should be, = 25.