Make an arbitrary change in the width of the slit. Do not make the width too large, because Fraunhofer theory will fail in that case, but make it sufficiently different that the pattern seen in the microscope is appreciably altered. Measure the positions of the first zeros on each side of the central maximum. Calculate the width of the slit. Move the slit forward again until it is in sharp focus, and measure the slit width with the microscope. Compare the measured slit width to the width calculated from the diffraction pattern.